patman: Add "postfix" support to patch subjects

In some communities, it may be necessary to append something after PATCH
in the subject line. For example, the Linux networking subsystem
expects [1] patch subject prefixes like [RFC PATCH net-next 0/99]. This
adds support for such "postfix"s to patman. Although entirely cosmetic,
it is still nice to have.

[1] https://www.kernel.org/doc/html/latest/networking/netdev-FAQ.html#how-do-i-indicate-which-tree-net-vs-net-next-my-patch-should-be-in



Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>