MFC r305855, r306297, r306300, r306312-r306313

When MAKEOBJDIRPREFIX points to a case-insensitive file system, the
build can break when different source files create the same object
files (case-insensitivity speaking). This is the case for object
files compiled with -fpic and shared libraries. The former uses
an extension of ".So", and the latter an extension ".so". Rename
shared object files from *.So to *.pico to match what NetBSD does.

Also:
o Compile _Exit.c as C99_Exit.c, as it conflicts with _exit.s
o Add entry to UPDATING
o Document .pico extension

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Don't strip paths away from META_XTRAS.

This would otherwise disallow using meta files from a foreign build that
spread them around in directories outside our own .OBJDIR.

Sponsored by: EMC / Isilon Storage Division

Update dirdeps.mk et al

dirdeps.mk: move logic to handle -f dirdeps.mk to inside check
for first read of dirdeps.mk

Also fix handling of WITHOUT_DIRDEPS_BELOW

gendirdeps.mk: pass M2D_EXCLUDES to meta2deps

meta.autodep.mk: if we build with nofilemon, leave a cookie to
prevent updating dependencies until cleaned.

Reviewed by: bdrewery

Merge bmake-20160307

META MODE: Don't add staged DPADDs to DIRDEPS when bootstrapping gendirdeps.

This was finding libraries that were installed into DESTDIR/usr/lib,
where DESTDIR is the stage directory, and then adding in usr/lib to
DIRDEPS. Just exclude the STAGE_ROOT if defined.

Discussed with: sjg
Sponsored by: EMC / Isilon Storage Division

META MODE: Define variables this file uses that were defined elsewhere.

Discussed with: sjg
Sponsored by: EMC / Isilon Storage Division

META MODE: Don't truncate DIRDEPS for bootstrapping during gendirdeps.

Reviewed by: sjg
Sponsored by: EMC / Isilon Storage Division

Add META_MODE support.

Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.

Still need to add real targets under targets/ to build packages.

Differential Revision: D2796
Reviewed by: brooks imp