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