ocaml: rename "sa_u" to "saddr_u"

<signal.h> in POSIX reserves the "sa_" prefix:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_02_02



Let's use "saddr_" instead.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20230215141158.2426855-5-lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>