Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r246119:
Rework the handling of the children for the pthread_vfork_test. The
trivial handler for SIGCHLD is installed, and SIGCHLD is blocked, to
not abandon our zombies to init(8). This way, the zombies are around
slightly longer, allowing to actually exercise the logic for p_pwait
use by the test.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

This is simple testing program for revision 185647.
It invokes multiple parallel threads and each thread calls vfork()
system call.

Approved by: kib