parse-maintainers: Use perl hash references and specific filenames

Instead of reading STDIN and writing STDOUT, use specific filenames of
MAINTAINERS and MAINTAINERS.new.

Use hash references instead of global hash %hash so future modifications
can read and write specific hashes to split up MAINTAINERS into multiple
files using a script.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>