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

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

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

copy and paste error

ufds might not be allocated in case nfds overflows to zero while
extra_nfds is still non-zero. udfs is then accessed within the
extra_nfds-based for loop.

Follow up on b0e74254

In case the first address in the tempaddr array is NULL,
the code would previously dereference an unchecked null pointer.

I was tempted to change those to >= 0 until I saw that this is
actually a for loop that terminates once i underflows.

The FreeContextBuffer SAL declaration does not declare the pointer
as optional, therefore it must not be NULL.

Added support for parsing query strings from the URL as defined by
RFC-5092.