Port memory-full checking to GnuTLS 3.3

Instead of using gnutls_global_set_mem_functions, check every call
to a GnuTLS function that might return an indication of memory
exhaustion.  Suggested by Dmitry Antipov in:
http://lists.gnu.org/archive/html/emacs-devel/2014-12/msg02056.html
* src/gnutls.c (gnutls_global_set_mem_functions) [WINDOWSNT]: Remove.
(init_gnutls_functions): Do not load gnutls_global_set_mem_functions.
(fn_gnutls_global_set_mem_functions) [!WINDOWSNT]: Remove.
All uses removed.
(check_memory_full): New function.
(emacs_gnutls_handshake, emacs_gnutls_handle_error)
(gnutls_make_error, Fgnutls_boot): Use it.
(emacs_gnutls_global_init): Avoid gnutls_global_set_mem_functions.