file_case: support transparent file decompression..
Since now file decompression can optionally be supported for all
files which get opened via file_case{,::muxer()} this changeset in
effect adds support for *all* file being compressed.
(It depends on file_case::_mux_unpack_default if this is actually
true -- it is mux_unpack now, but changing it to ::mux_no_unpack
will reduce compression support to those facilities which
*explicitly* specify mux_unpack, which this changeset only uses
for the `.so' request.)
Inspired by a commit from Matthew Dillon (dillon AT apollo DOT
backplane DOT com) of DragonFly BSD ([
84d7c06], 2014-05-20).