fix closing of fd's when limit is reached

when more than FD_SETSIZE file descriptors are open.
This means that if for any reason we are not able to
open more than FD_SETSIZE file descriptors then test
518 should not be run.

test 537 is all about testing libcurl functionality
when the system has nearly exhausted the number of
free file descriptors. Test 537 will try to run with
very few free file descriptors.

In this way we'll be able to sort out problems that might
arise in the prechek phase of the 518 test.

Once that 518 has been verified this change will be undone.

preventing execution of this test on many platforms

on Win32.

- Take in account RLIM_INFINITY.
- Verify that soft limit is actually changed when doing so.
- Show errno in case getrlimit or setrlimit fails.
- Keep file descriptors open only while runing this test.

case when 401 or 407 are returned, *IF* no auth credentials have been given.
The CURLOPT_FAILONERROR option is not possible to make fool-proof for 401
and 407 cases when auth credentials is given, but we've now covered this
somewhat more.

You might get some amounts of headers transferred before this situation is
detected, like for when a "100-continue" is received as a response to a
POST/PUT and a 401 or 407 is received immediately afterwards.

Added test 281 to verify this change.

Improve cleanup in case of initialization failure.

test hanging and actually is an indication that there's a condition that is
not being properly handled at some point in the library.

Remove a pair of braces and adjust indentation appropriately.

the single test applications' link and dependences, so that you easier can
override those from the command line when using make.

reading the (local) CA cert file to let users easier pinpoint the actual
problem. CURLE_SSL_CACERT_BADFILE (77) is the new libcurl error code.

stale by replacing loop counters with timeouts. In this way the
main loop of the test will be allowed to run up to 30 seconds on
any platform before aborting it.

test hanging and actually is an indication that there's a condition that is
not being properly handled at some point in the library.

Loop counter limits might need to be further increased on false positives.

to verify winsock API availability.

present, so there's no use to doing it again in this script!