use liblzma-dev to provide xz/lzma support

use liblzma-dev to provide xz/lzma support

We have xz/lzma support for a while, but only via an external binary
provided by xz-utils. Now that the Debian archive provides xz by default
and dpkg pre-depends on the library provided by liblzma-dev we can switch
now to use this library as well to avoid requiring an external binary.
For now the binary is in a prio:required package, but this might change
in the future.

API wise it is quiet similar to bz2 code expect that it doesn't provide
file I/O methods, so we piece this together on our own.