Fix glitches in gnutls.c, mostly memory-related

* gnutls.c: Sort macro definitions by name.
(fn_gnutls_transport_set_errno): Omit unreachable definition.
(fn_gnutls_x509_crt_get_signature): Omit unused symbol.
(gnutls_hex_string): Arg is now unsigned char *, to avoid a cast.
Prefer ptrdiff_t for sizes.  Check for arithmetic overflow when
calculating string length.  Use make_uninit_string, to avoid
copying the string.  Cast the char, not the pointer.
(gnutls_certificate_details): Use xmalloc and xfree, not malloc
and free.  Work even for dates past the year 9999.  Use void *
for buffers, to avoid casts.
* automated/vc-tests.el (vc-test--register): Check, that the file
still exists after register and unregister operations.