lib/util: Avoid splitting tevent-unix-util as public library

lib/util: Avoid splitting tevent-unix-util as public library

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11946

Commit 670db6ac1d678babd25dd82c4467c0f094cfabc5 split tevent-util public
library to create tevent-unix-util public library for standalone ctdb
use.  This created a public library dependency between samba and ctdb
for packaging.

Bundle tevent_unix.c in public library tevent-util as before.  However,
to avoid the dependencies for packaging, standalone ctdb build will
build tevent-util as a private library with only tevent_unix.c

This simplifies any new subsystems (or libraries) which need tevent-util
and are linked in both samba and ctdb.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Volker Lendecke <vl@samba.org>