scripts/get_maintainer.pl: optionally ignore non-maintainer signatures

When using --git to determine who to send a patch to, get_maintainers will
currently include all signatures.  This can include signers that simply
report an issue or test a patch.  Signers that use this tag are not
necessarily good candidates to review new patches.

This patch allows get_maintainers to optionally restrict output to only
signatures that use Signed-off-by:, Acked-by:, or Reviewed-by:.

Signed-off-by: is included because those are people who are responsible
for the code.

Acked-by: is questionable, but as signers that use this tag tend to be
active linux gatekeepers, false positives are tolerable.

Reviewed-by: is included because signers responsible for the code thought
that the review feedback for a changeset by that signer was valuable.

This patch has been modified from Florian's original submission to change
the supported signature types to the canonical forms and use slightly
different spacing.  A couple of spacing issues were also corrected in the
original source.  The command line argument was also renamed.

Original-patch-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Florian Mickler <florian@mickler.org>
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>