xfs_repair: add flag -e to modify exit code for corrected errors

xfs_repair: add flag -e to modify exit code for corrected errors

xfs_repair without -n ends with a return code 0 if it finished ok,
no matter if there were some errors in the fs, or not. The new flag
-e means that we can avoid screenscraping and parsing text output to
detect if an error was found (and corrected).

If something could not be corrected or in any other case than the "found
something but fixed it all," the behaviour with this flag is unchanged.

Signed-off-by: Jan Tulak <jtulak@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
[sandeen: make -e and -n exclusivity clear in manpage synopsis]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>