sbappendcontrol() needs to avoid clearing M_NOTREADY on data mbufs.

If LOCAL_CREDS is set on a unix socket and sendfile() is called,
sendfile will call uipc_send(PRUS_NOTREADY), prepending a control
message to the M_NOTREADY mbufs. uipc_send() then calls
sbappendcontrol() instead of sbappend(), and sbappendcontrol() would
erroneously clear M_NOTREADY.

Pass send flags to sbappendcontrol(), like we do for sbappend(), to
preserve M_READY when necessary.

Reported by: syzkaller
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24333