Fix compiler warning: variable was set but never used

Fix compiler warning: clobber ignored

By the use of a the new lib/checksrc.pl script that checks that our
basic source style rules are followed.

POLLIN, and sets POLLERR without setting POLLIN and POLLOUT. In some
libcurl code execution paths this could trigger busy wait loops with
high CPU usage until a timeout condition aborted the loop.

This fix for Curl_poll adresses the above in a libcurl-wide mode.

This also implies the removal of the winmm.lib dependency for WIN32.

select/poll calls will only be retried upon EINTR failures as
it previously was in lib/select.c revision 1.29

In this way Curl_socket_ready() and Curl_poll() will again fail
on any select/poll errors different than EINTR.

consistency

to find that it crashed miserably, and this was due to some select()isms left
in the code. This was due to API restrictions in c-ares 1.3.x, but with the
upcoming c-ares 1.4.0 this is no longer the case so now libcurl runs much
better with c-ares and the multi interface with > 1024 file descriptors in
use.

I also switched from calloc() to malloc() as a minor performance boost since
the rest of the code fills in the structs fine anyway - and they must for the
case when we use the stack-based auto variable array instead of the allocated
one.

I made the loop filling in poll_fds[] break when poll_nfds is reached as a
minor speed improvement.

'pending_ms' may be used uninitialized in this function

  Curl_socket_ready(), Curl_poll() and Curl_select() when these are called
  with a zero timeout or a timeout value indicating a blocking call should
  be performed.

  These unnecessary calls to gettimeofday() got introduced in 7.16.2 when
  fixing 'timeout would restart when signal caught while awaiting socket
  events' on 20 March 2007.

- Move some loop breaking logic from the while clause into the loop,
  avoiding compiler warning 'assignment within conditional expression'

  function that deprecates the curl_multi_socket() function. Using the new
  function the application tell libcurl what action that was found in the
  socket that it passes in. This gives a significant performance boost as it
  allows libcurl to avoid a call to poll()/select() for every call to
  curl_multi_socket*().

to complete with no delay and actually find out what happened with
the socket. As well as detection of socket send()able condition.

This also allows removal of a Cygwin specific block of code.

uses poll() when a fine poll() is available, so now libcurl can be
built without select() support at all if a fine poll() is available.

For completeness sake, wait_ms() might also get interrupted when
experimental CURL_ACKNOWLEDGE_EINTR is defined.

or Curl_poll() with a non-zero timeout both functions would restart the
specified timeout. This could even lead to the extreme case that if a
signal arrived with a frecuency lower to the specified timeout neither
function would ever exit.

Added experimental symbol definition check CURL_ACKNOWLEDGE_EINTR in
Curl_select() and Curl_poll(). When compiled with CURL_ACKNOWLEDGE_EINTR
defined both functions will return as soon as a signal is caught. Use it
at your own risk, all calls to these functions in the library should be
revisited and checked before fully supporting this feature.

Curl_poll() which is called whenever not a single valid file descriptor is
passed to these functions.

Improve readibility using a poll() macro to replace WSApoll().

  1) the progress callback gets called more frequently (at times)
  2) libcurl *might* call the callback when it receives a signal

since they're already included through "setup.h".

  (http://curl.haxx.se/bug/view.cgi?id=1603712) (known bug #36) --limit-rate
  (CURLOPT_MAX_SEND_SPEED_LARGE and CURLOPT_MAX_RECV_SPEED_LARGE) are broken
  on Windows (since 7.16.0, but that's when they were introduced as previous
  to that the limiting logic was made in the application only and not in the
  library). It was actually also broken on select()-based systems (as apposed
  to poll()) but we haven't had any such reports. We now use select(), Sleep()
  or delay() properly to sleep a while without waiting for anything input or
  output when the rate limiting is activated with the easy interface.

HTTP upload was disconnected:

"What appears to be happening is that my system (Linux 2.6.17 and 2.6.13) is
setting *only* POLLHUP on poll() when the conditions in my previous mail
occur. As you can see, select.c:Curl_select() does not check for POLLHUP. So
basically what was happening, is poll() was returning immediately (with
POLLHUP set), but when Curl_select() looked at the bits, neither POLLERR or
POLLOUT was set. This still caused Curl_readwrite() to be called, which
quickly returned. Then the transfer() loop kept continuing at full speed
forever."

to verify winsock API availability.

In this way 304 tests out of 304 reported OK.

Vista (_WIN32_WINNT >= 0x0600)