A debugger can modify a non-reporting LWP's registers while it remains
suspended in an interruptible syscall. The continue path only checks td2
for TDB_USERWR, so PT_CONTINUE addressed to the process can inspect the
reporting LWP instead and leave the modified LWP asleep indefinitely.
Check all modified LWPs eligible to resume, wake interruptible sleepers,
and clear TDB_USERWR only when aborting their sleep. Preserve modifications
on LWPs explicitly left suspended so their later resume handles the wakeup.
Keep the existing transparent-stop setting and sleepq locking protocol.
MFC after: 2 weeks
Sponsored by: FreeBSD Foundation