PT_ATTACH: do not interrupt interruptible sleeps

Handle the first SIGSTOP from the PT_ATTACH specially: instead of waking
up some thread in the target process, execute sig_suspend_threads() in
the debugger. This is equivalent to the action of the xthread, except
that all threads are suspended without interruption.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D49678