ctdb-common: Fix error handling when sending ARPs

ctdb-common: Fix error handling when sending ARPs

There are numerous places in the code where errno can be lost causing
the wrong error to be printed by a caller.  Change ctdb_sys_send_arp()
to always return a useful errno on error instead of returning -1 and
sometimes having errno set correctly.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>