Setgid before running a command as a specified user.  Previously only
initgroups(3) was called, what isn't quite enough.  This brings jail(8)
in line with jexec(8), which was already doing the right thing.

PR:		195984
MFC after:	1 week
Fix building/installing tests when TESTSBASE != /usr/tests

The work in r258233 hardcoded the assumption that tests was the last component
of the tests tree by pushing tests as an explicit prefix for the paths in
BSD.tests.dist and /usr was the prefix for all tests, per BSD.usr.dist and all
of the mtree calls used in Makefile.inc1. This assumption breaks if/when one
provides a custom TESTSBASE "prefix", e.g. TESTSBASE=/mytests .

One thing that r258233 did properly though was remove "/usr/tests" creation
from BSD.usr.dist -- that should have not been there in the first place. That
was an "oops" on my part for the work that was originally committed in r241823

MFC after: 2 weeks
Phabric: D1301
Reviewed by: imp
Sponsored by: EMC / Isilon Storage Division
Document STRIP_CMD in build(7) and note its importance with LOCAL_ITOOLS

MFC after: 1 week
Phabric: D1335
Reviewed by: brueffer
Sponsored by: EMC / Isilon Storage Division