This commit adds trailer support in HTTP/2.  In HTTP/1.1, chunked
encoding must be used to send trialer fields.  HTTP/2 deprecated any
trandfer-encoding, including chunked.  But trailer fields are now
always available.

Since trailer fields are relatively rare these days (gRPC uses them
extensively though), allocating buffer for trailer fields is done when
we detect that HEADERS frame containing trailer fields is started.  We
use Curl_add_buffer_* functions to buffer all trailers, just like we
do for regular header fields.  And then deliver them when stream is
closed.  We have to be careful here so that all data are delivered to
upper layer before sending trailers to the application.

We can deliver trailer field one by one using NGHTTP2_ERR_PAUSE
mechanism, but current method is far more simple.

Another possibility is use chunked encoding internally for HTTP/2
traffic.  I have not tested it, but it could add another overhead.

Closes #564

- In Curl_verifyhost check all altnames in the certificate.

Prior to this change only the first altname was checked. Only the GSKit
SSL backend was affected by this bug.

Bug: http://curl.haxx.se/mail/lib-2015-12/0062.html
Reported-by: John Kohl

This is the new command line option to set the value for the existing
libcurl option CURLOPT_EXPECT_100_TIMEOUT_MS

Closes #565

Make this the default for the curl tool (if built with HTTP/2 powers
enabled) unless a specific HTTP version is requested on the command
line.

This should allow more users to get HTTP/2 powers without having to
change anything.

... and stick to 1.1 for HTTP. This is in line with what browsers do and
should have very little risk.

Closes #491

Closes #555

Reported-by: Thomas Klausner

When NGHTTP2_ERR_PAUSE is returned from data_source_read_callback, we
might not process DATA frame fully.  Calling nghttp2_session_mem_recv()
again will continue to process DATA frame, but if there is no incoming
frames, then we have to call it again with 0-length data.  Without this,
on_stream_close callback will not be called, and stream could be hanged.

Bug: http://curl.haxx.se/mail/lib-2015-11/0103.html
Reported-by: Francisco Moraes

With curl disable verbose strings in http.c the compilation fails due to
the data variable being undefined later on in the function.

Closes #558

The name of the header guard in lwIP's <lwip/opt.h> has changed from
'__LWIP_OPT_H__' to 'LWIP_HDR_OPT_H' (bug #35874 in May 2015).

Other fixes:

- In curl_setup.h, the problem with an old PSDK doesn't apply if lwIP is
used.

- In memdebug.h, the 'socket' should be undefined first due to lwIP's
lwip_socket() macro.

- In curl_addrinfo.c lwIP's getaddrinfo() + freeaddrinfo() macros need
special handling because they were undef'ed in memdebug.h.

- In select.c we can't use preprocessor conditionals inside select if
MSVC and select is a macro, as it is with lwIP.

http://curl.haxx.se/mail/lib-2015-12/0023.html
http://curl.haxx.se/mail/lib-2015-12/0024.html

- If the size of the length type (curl_off_t) is greater than the size
of the size_t type then check before allocating memory to make sure the
value of length will fit in a size_t without overflow. If it doesn't
then return CURLE_BAD_FUNCTION_ARGUMENT.

Bug: https://github.com/bagder/curl/issues/425#issuecomment-154518679
Reported-by: Steve Holme

They didn't match the ifdef logic used within libcurl anyway so they
could indeed warn for the wrong case - plus the tool cannot know how the
lib actually performs at that level.

Tests 842, 843, 844, 845, 887, 888, 889, 890, 946, 947, 948 and 949 fail
if a custom port number is specified via the -b option of runtests.pl.

Suggested by: Kamil Dudka
Bug: http://curl.haxx.se/mail/lib-2015-12/0003.html

for all we know now, it might be called 7.46.1

Closes #546

- set the correct stream_id for pushed streams
- init maxdownload and size properly