restore: Add regex matching of paths and files to be restored

restore: Add regex matching of paths and files to be restored

The option -m is used to specify the regex string. -c is used to
specify case insensitive matching. -i was already taken.

In order to restore only a single folder somewhere in the btrfs
tree, it is unfortunately neccessary to construct a slightly
nontrivial regex, e.g.:

restore -m '^/(|home(|/username(|/Desktop(|/.*))))$' /dev/sdb2 /output

This is needed in order to match each directory along the way to the
Desktop directory, as well as all contents below the Desktop directory.

Signed-off-by: Peter Stuge <peter@stuge.se>
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>