MAINTAINERS: update the SELinux entry

Bring the SELinux entry up to date with the following changes:

* Remove the selinuxproject.org link.  The wiki located there is in
read-only mode and exists primarily for historical reasons.

* Add our patchwork link.  I'm not sure this is of much use for
anyone but the maintainer, but there is a provision for including it
here so we might as well include it.

* Add a bug report URI.  I suspect most everyone knows to send mail
to the mailing list if they hit a bug, but let's make it official.

* Add a link to the SELinux tree process/management documentation.
While the doc exists both in the canonical kernel.org location and
the GitHub mirror, provide a link to the mirror as GitHub does a
better job rendering the Markdown.

* Update the source tree's git URI to use https.

Signed-off-by: Paul Moore <paul@paul-moore.com>