fsfreeze: add --auto-unfreeze <timeout>

This is a little bit controversial, because all this functionally
cloud be implemented by careful shell scripting, but it seems that
for some users it's more attractive to have a robust solution within
fsfreeze(8) than depend on scripts with something so critical like
unaccessible filesystems.

References: https://bugzilla.redhat.com/show_bug.cgi?id=880365
Signed-off-by: Karel Zak <kzak@redhat.com>