P2_WEXIT: avoid thread_single() for exiting process earlier

before the process itself does thread_single(SINGLE_EXIT). We cannot
single-thread such process in ALLPROC (external) mode, and properly
detect and report the failure to do so due to the process becoming
zombie is easier to prevent than handle.

In collaboration with: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D35310