MFC r314659,r314676:

r314659:

usr.sbin: normalize paths using SRCTOP-relative paths or :H when possible

This simplifies make logic/output

r314676:

Fix build after r314656

Some of the changes I introduced to use .ALLSRC were correct in spirit,
but incorrect in reality -- in particular, ../Makefile.inc hadn't been
pulled in via bsd.init.mk (via bsd.lib.mk, bsd.prog.mk), so the value
of .ALLSRC (evaluated immediately) was empty. .include bsd.init.mk
explicitly so we can be certain that the values used as dependencies in
the targets are defined when the target recipe has been evaluated.

Reminder: thou shalt separate out separate functional changes before
committing them.

(YUGE) Pointyhat to: ngie
In collaboration with: bdrewery

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

Convert usr.sbin to LIBADD
Reduce overlinking

Start committing Bluetooth HID (Human Interface Device) support.
Note: bthidd(8) is still not complete. Need to commit kernel
support (a-la Linux /dev/input) to feed HID events into kernel.
Also need to write bthidd(8) and bthidd.conf(5) man pages.