Fix gnutls tls_socket_write() to properly retry

Fix gnutls tls_socket_write() to properly retry

Retry on GNUTLS_E_AGAIN and GNUTLS_E_INTERRUPTED.  This prevents an
aborted send due to a SIGWINCH, for instance.

Change tls_socket_read() to follow the same flow.  Don't bother
checking gnutls_error_is_fatal() because return codes besides AGAIN
and INTERRUPTED end up closing the connection regardless.  (We don't
handle handshakes and negotations during send/receive).

Co-authored-by: Richard Russon <rich@flatcap.org>