blkid: add pretty output, document -L incompatibility with e2fsprogs

blkid: add pretty output, document -L incompatibility with e2fsprogs

... sad story, I have temporary disabled pretty-output code
in very early version of blkid.c in u-l-ng. (It was also in
time when pretty-output was very new feature in e2fsprogs.)

Unfortunately, the -L option (shortcut to "-o list") in u-l-ng version
was reused for for any other functionality few months later.... this
stupid thing was released in u-l-ng 2.15 and 2.16 without any negative
feedback from users.

It means the blkid from u-l-ng is not backwardly compatible with
the original version from e2fsprogs. The -L option has a different
meaning there.

I'm sorry about this bug...

This patch:

  * enable pretty-output (-o line)
  * add a note about incompatibility between u-l-ng and e2fsprogs
    to blkid.8 man page

Signed-off-by: Karel Zak <kzak@redhat.com>