Closes #956

Follow-up to e577c43b to fix test case 569 brekage: stop the parser at
whitespace as well.

Help-by: Erik Janssen

Mark's new document about HTTP Retries
(https://mnot.github.io/I-D/httpbis-retry/) made me check our code and I
spotted that we don't retry failed HEAD requests which seems totally
inconsistent and I can't see any reason for that separate treatment.

So, no separate treatment for HEAD starting now. A HTTP request sent
over a reused connection that gets cut off before a single byte is
received will be retried on a fresh connection.

Made-aware-by: Mark Nottingham

Makes libcurl work in communication with gstreamer-based RTSP
servers. The original code validates the session id to be in accordance
with the RFC. I think it is better not to do that:

- For curl the actual content is a don't care.

- The clarity of the RFC is debatable, is $ allowed or only as \$, that
  is imho not clear

- Gstreamer seems to url-encode the session id but % is not allowed by
the RFC

- less code

With this patch curl will correctly handle real-life lines like:
Session: biTN4Kc.8%2B1w-AF.; timeout=60

Bug: https://curl.haxx.se/mail/lib-2016-08/0076.html

Added in 5fce88aa

This makes it possible to use specific compilers or a cache.

Sample use for clcache:
set CC=clcache.bat
nmake /f Makefile.vc DEBUG=no MODE=static VC=14 GEN_PDB=no

In the old line number 290, CC and CURL_CC had the same value. After
that, /DCURL_STATICLIB was added to CC but not CURL_CC (intended?).

This gets rid of the CC variable entirely. It is a first step to make it
possible to manualyl set a CC variable in order to be able to change the
compiler.

$(CURL_CC) is always used with $(CURL_CFLAGS) appended, so before this,
all arguments in CURL_CFLAGS have been added twice.

- Turn on USE_THREADS_WIN32 in Windows if ares isn't on

This change is similar to what we already do in the autotools build.

All compilers used by cmake in Windows should support large files.

- Add test SIZEOF_OFF_T
- Remove outdated test SIZEOF_CURL_OFF_T
- Turn on USE_WIN32_LARGE_FILES in Windows
- Check for 'Largefile' during the features output

Since the server can at any time send a HTTP/2 frame to us, we need to
wait for the socket to be readable during all transfers so that we can
act on incoming frames even when uploading etc.

Reminded-by: Tatsuhiro Tsujikawa

In order to make MBEDTLS_DEBUG work, the debug threshold must be unequal
to 0.  This patch also adds a comment how mbedtls must be compiled in
order to make debugging work, and explains the possible debug levels.

After a few wasted hours hunting down the reason for slowness during a
TLS handshake that turned out to be because of TCP_NODELAY not being
set, I think we have enough motivation to toggle the default for this
option. We now enable TCP_NODELAY by default and allow applications to
switch it off.

This also makes --tcp-nodelay unnecessary, but --no-tcp-nodelay can be
used to disable it.

Thanks-to: Tim Rühsen
Bug: https://curl.haxx.se/mail/lib-2016-06/0143.html

When input stream for curl is stdin and input stream is not a file but
generated by a script then curl can truncate data transfer to arbitrary
size since a partial packet is treated as end of transfer by TFTP.

Fixes #857

Makes the script pass on comments holding meta data to the output
file. Like fingerprinters, issuer, date ranges etc.

Closes #937

Previously, passing a timeout of zero to Curl_expire() was a magic code
for clearing all timeouts for the handle. That is now instead made with
the new Curl_expire_clear() function and thus a 0 timeout is fine to set
and will trigger a timeout ASAP.

This will help removing short delays, in particular notable when doing
HTTP/2.

Regression added in 790d6de4. The was then added to avoid one
particular transfer to starve out others. But when aborting due to
reading the maxcount, the connection must be marked to be read from
again without first doing a select as for some protocols (like SFTP/SCP)
the data may already have been read off the socket.

Reported-by: Dan Donahue
Bug: https://curl.haxx.se/mail/lib-2016-07/0057.html

Fixes #943