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
Don't build full clang toolchain or clang extras in stages 1-3 of buildworld

MFC after: 1 week
Reviewed by: dim (as part of a "larger" diff)
Phabric: D1336
Sponsored by: EMC / Isilon Storage Division
Fix accidental MK_DEBUG_FILES=no addition to NXBMAKE in r275909

X-MFC with: r275909
Sponsored by: EMC / Isilon Storage Division
Include section name in strip warning message
Sync with NetBSD, mainly address NetBSD bug #43355:

Fix valid_format() to be more careful about allowing only valid
printf formats.

Obtained from:	NetBSD
MFC after:	2 weeks
Pass real optimal transfer size supported by backend.

For files and ZVOLs that is 1MB now, not 128K.

MFC after:	1 week