Require libblkid

Historically libblkid support was detected as part of configure
and optionally enabled. This was done because at the time support
for detecting ZFS pool vdevs had just be added to libblkid and
those updated packages were not yet part of many distributions.
This is no longer the case and any reasonably current distribution
will ship a version of libblkid which can detect ZFS pool vdevs.

This patch makes libblkid mandatory at build time and libblkid
the preferred method of scanning for ZFS pools. For distributions
which include a modern version of libblkid there is no change in
behavior. Explicitly scanning the default search paths is still
supported and can be enabled with the '-s' command line option.

Additionally making libblkid mandatory means that the 'zpool create'
command can reliably detect if a specified device has an existing
non-ZFS filesystem (ext4, xfs) and print a warning.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2448