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Without this change, the gnutls library would invoke malloc etc.
directly, which causes problems on non-SYNC_INPUT hosts, and which
runs afoul of improving memory_full behavior.
(fn_gnutls_global_set_mem_functions): New macro or function pointer.
(emacs_gnutls_global_init): Use it to specify xmalloc, xrealloc,
xfree instead of the default malloc, realloc, free.
(Fgnutls_boot): No need to check for memory allocation failure,
since xmalloc does that for us.