Fix raw socket read/write to follow expected behavior

Fix raw socket read/write to follow expected behavior

The mutt_sasl.c code expects conn_write() to write the entire buffer.
This is inconsistent with mutt_socket.c, but since other conn_write()
implementations guarantee this, change raw_socket_write() to do so too
for now.

Also, update reading and writing to loop on EINTR, as gnutls does.
They won't return EAGAIN or EWOULDBLOCK because we don't mark sockets
as non-blocking.

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