sigqueue(2): add impl-specific flag __SIGQUEUE_TID

The flag allows the pid argument to designate a thread from the calling
process. The flag value is carved from the high bit of the signal
number, which slightly changes the ABI of syscall.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44867