use ssize_t and size_t for read- and write-like emacs_gnutls_* functions

use ssize_t and size_t for read- and write-like emacs_gnutls_* functions

* gnutls.c (emacs_gnutls_read): Adjust signature to be more read-like:
return ssize_t not "int", and use size_t as the buffer length.
(emacs_gnutls_write): Likewise, and make the buffer pointer "const".
* gnutls.h: Update declarations.
* process.c (read_process_output): Use ssize_t, to match.
(send_process): Likewise.