get_maintainer: allow option --mpath <directory> to read all files in <directory>

There is an external use case for multiple private MAINTAINER style files
in a separate directory.  Allow it.

--mpath has a default of "./MAINTAINERS".

The value entered can be either a file or a directory.

The behaviors are now:

--mpath <file>          Read only the specific file as <MAINTAINER_TYPE> file
--mpath <directory>     Read all files in <directory> as <MAINTAINER_TYPE> files
--mpath <directory> --find-maintainer-files
                        Recurse through <directory> and read all files named MAINTAINERS

Link: http://lkml.kernel.org/r/991b2f20112d53863cd79e61d908f1d26d3e1971.camel@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Tested-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>