scripts/get_maintainer.pl: add ability to read from STDIN

Doesn't need or accept '-' as a trailing option to read stdin.  Doesn't
print usage() after bad options.  Adds --usage as command line equivalent
of --help

Suggested-by: Borislav Petkov <petkovbb@googlemail.com>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>