configure: use the nghttp2 path correctly with pkg-config

When --with-nghttp2 was used (without a given path), the
PKG_CONFIG_LIBDIR varialbe could get clobbered and ruin a proper
detection of the library.

Reported-by: Dilyan Palauzov
Bug: http://curl.haxx.se/mail/lib-2014-04/0159.html
sockfilt.c: fixed getting stuck waiting for MinGW stdin pipe
sockfilt.c: zero initialize variable
sockfilt.c: clean up threaded approach and add documentation
tests: Fixed torture test for tests 1526 & 1527
nss: split Curl_nss_connect() into 4 functions
nss: implement non-blocking SSL handshake
curl_global_init_mem: bump initialized even if already initialized

As this makes curl_global_init_mem() behave the same way as
curl_global_init() already does in that aspect - the same number of
curl_global_cleanup() calls is then required to again decrease the
counter and then eventually do the cleanup.

Bug: http://curl.haxx.se/bug/view.cgi?id=1362
Reported-by: Tristan
gtls: fix NULL pointer dereference

gnutls_x509_crt_import() must not be called with a NULL certificate

Bug: http://curl.haxx.se/mail/lib-2014-04/0145.html
Reported-by: Damian Dixon
RELEASE-NOTES: Synced with 386ed2d590
cyassl: Use error-ssl.h when available

Versions since at least 2.9.4 renamed error.h to error-ssl.h, so use
whichever one is available.
build: Added Visual Studio 2013 (VC12) project files

Carrying on from commit 11025613b9 added VC12 project files which are
capable of supporting side-by-side compilation, 32-bit and 64-bit
builds as well as support for some of the third-party libraries curl
uses.
handler: make 'protocol' always specified as a single bit

This makes the findprotocol() function work as intended so that libcurl
can properly be restricted to not support HTTP while still supporting
HTTPS - since the HTTPS handler previously set both the HTTP and HTTPS
bits in the protocol field.

This fixes --proto and --proto-redir for most SSL protocols.

This is done by adding a few new convenience defines that groups HTTP
and HTTPS, FTP and FTPS etc that should then be used when the code wants
to check for both protocols at once. PROTO_FAMILY_[protocol] style.

Bug: https://github.com/bagder/curl/pull/97
Reported-by: drizzt
test325: verify --proto-redir https=>http
nss: propagate blocking direction from NSPR I/O

... during the non-blocking SSL handshake