Fix fstab block device resolution for FreeBSD

The device name prefix for IDE hard drives used to be `ad' but now
is `ada' (http://www.freebsd.org/doc/handbook/disks-naming.html).
For virtio hard drives it is `vtbd'.

Under an MBR partition table a slice will be used, so the name of
the first partitions will be either `ada0s1a' or `vtbd0s1a'. Under a
GPT partition table, where no slice is needed, the name of the first
partition will be either `ada0p1' or `vtbd0p1'.

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>
builder: Internal implementation of parallel xzcat (pxzcat).

Instead of calling out to the pxzcat program, use an internal
implementation.  This requires liblzma to be available at build time.
If it's not available, fall back to using regular xzcat.

It is intended that eventually this code will go away when regular
xzcat / unxz is able to use threads.