Let ptracestop() suspend threads sleeping in an SBDRY section.

When a thread enters ptracestop(), for example because it had received
SIGSTOP from ptrace(PT_ATTACH), it attempts to suspend other threads in
the same process. In the case of a thread sleeping interruptibly in an
SBDRY section, sig_suspend_threads() must wake the thread and allow it to
reach the user-mode boundary. However, sig_suspend_threads() would
erroneously avoid waking up such threads, resulting in an apparent hang.

Reviewed by: kib
Tested by: pho
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon