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

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.

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.

Should a command return untagged responses that contained no data then
the imap_matchresp() function would not detect them as valid responses,
as it wasn't taking the CRLF characters into account at the end of each
line.

Given that we presently support "auth" and not "auth-int" or "auth-conf"
for native challenge-response messages, added client side validation of
the quality-of-protection options from the server's challenge message.

Bug: http://curl.haxx.se/mail/lib-2014-04/0063.html
Reported-by: Jeroen Koekkoek

To avoid urldata.h being included from the header file or that the
source file has the correct include order as highlighted by one of
the auto builds recently.

warning: no previous prototype for 'Curl_sasl_create_digest_md5_message'

warning: 'sasl_digest_get_key_value' defined but not used

* Added comments to SSPI NTLM message generation
* Added comments to native DIGEST-MD5 code
* Removed redundant identity pointer

When CURL_DISABLE_CRYPTO_AUTH is defined the DIGEST-MD5 code should not
be included, regardless of whether USE__WINDOWS_SSPI is defined or not.
This is indicated by the definition of USE_HTTP_NEGOTIATE and USE_NTLM
in curl_setup.h.