sys/acpi: Bring in Intel's acpica-unix2-20131218 verbatim.

On acpica.org, -unix is the version with an Intel license and
-unix2 is the BSD/GPL dual licensed version. I didn't want to
call the directory acpica-unix2 instead of our acpica-unix so
I settled on simply acpica.

Further updates would be done in-place in that directory.
acpi: Fix building userland.

As of this commit we're losing acpidb(8) which gets replaced by
acpiexec(8).

Additionally, we're getting:

acpibin(8)
acpiexec(8)
acpihelp(8)
acpinames(8)
acpisrc(8)
acpixtract(8)

Manual pages for the new utilities have yet to be written. Also
acpidump.8 and iasl.8 need to be updated, I guess.
acpi: Fix building the kernel.
Remove previous ACPICA.
kernel/acpi: Silence exception warning when a \\_Sx_ object can't be found.

Submitted-by: sephe