Modularize xz.

Modularize xz.

Embedded lzma decompression library becomes a module usable by other
consumers, in addition to geom_uzip.

Most important code changes are
- removal of XZ_DEC_SINGLE define, we need the code to work
  with XZ_DEC_DYNALLOC;
- xz_crc32_init() call is removed from geom_uzip, xz module handles
  initialization on its own.

xz is no longer embedded into geom_uzip, instead the depend line for
the module is provided, and corresponding kernel option is added to
each MIPS kernel config file using geom_uzip.

The commit also carries unrelated cleanup by removing excess "device geom_uzip"
in places which were missed in r344479.

Reviewed by:	cem, hselasky, ray, slavash (previous versions)
Sponsored by:	Mellanox Technologies
Differential revision:	https://reviews.freebsd.org/D19266
MFC after:	3 weeks