blkid: Automatically chose between ext4 and ext4dev as appropriate

blkid: Automatically chose between ext4 and ext4dev as appropriate

Add logic that on Linux systems will check for the presence of the
ext4dev filesystem; if it isn't present, fall back to ext4 for
filesystems that are marked as being "OK for use on test filesystem
code".  If they are OK for use for in-development filesystem code, it
should also be fine to use stable filesystem code if there is no test
filesystem code (ext4dev) available.

The reverse is not true, of course.  We don't ever want to mount a
production filesystem using test filesystem code unless the user gives
us explicit permission via "tune2fs -E test_fs".

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>