lib/tsocket: workaround sockets not supporting FIONREAD

lib/tsocket: workaround sockets not supporting FIONREAD

Netlink sockets don't support querying pending bytes with ioctl(fd,
FIONREAD, ...) and would return EOPNOTSUPP, so use recvmsg() with
MSG_PEEK|MSG_TRUNC as a fallback.

The MSG_TRUNC flag to recvmsg() is Linux only, but netlink is as well,
so we're safe for now.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11714

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Wed Feb 10 10:30:24 CET 2016 on sn-devel-144