Expose temporary mounts in mtab for nilfs resizing.
Add support for resizing volumes.
Increase test coverage.

The two largest untested sections are now
btrfs and reiserfs.  There is a blkid issue
with the former, and the latter insists on
interactivity when shrinking.
tests: Increase btrfs fs size, this solves the blkid issue.
Test reiserfs growth and xfs unshrinkability.

Coverage is now pretty good (94% with a bit of dead code).
Move tests.
Test in Travis.
tests: Simplify write access to /etc.