killpg(): close a race with fork(), part 2

When we are sending terminating signal to the group, killpg() needs to
guarantee that all group members are to be terminated (it does not need
to ensure that they are terminated on return from killpg()). The
pg_killsx change eliminates the largest window there, but still, if a
multithreaded process is signalled, the following could happen:

• thread 1 is selected for the signal delivery and gets descheduled
• thread 2 waits for pg_killsx lock, obtains it and forks
• thread 1 continue executing and terminates the process

This scenario allows the child to escape still.

To fix it, count the number of signals sent to the process with
killpg(2), in p_killpg_cnt variable, which is incremented in killpg()
and decremented after signal handler frame is created or in exit1()
after single-threading. This way we avoid forking if the termination is
due.

Noted and reviewed by: markj (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D40493