[Mailmunge] Spec file to build Mailmunge RPMS

[Kenneth Porter]

Good news. rpmbuild is fine with the spec file in a subdirectory.

I added it to DIST-CONTENTS and added " --add-file=spec/mailmunge.spec" 
to the git archive command in tools/make-dist.sh and get a tarball that 
rpmbuild can use.

The git archive command is a little funny because --add-file strips the 
path from the additional file and --add-prefix can be used multiple 
times to restore it and then set the prefix for the versioned files.

https://stackoverflow.com/questions/70512088/git-archive-adding-not-versioned-file-to-subdirectory

The resulting git command:

git archive --worktree-attributes --format=tar 
--prefix=mailmunge-$VERSION/spec/ --add-file=spec/mailmunge.spec 
--prefix=mailmunge-$VERSION/ HEAD | gzip -9 > mailmunge-$VERSION.tar.gz