make: Release pipeline

This commit adds new make targets for rpm, deb, and pacman packages.
It also adds a phony target that uploads tarballs of the packages,
along with their signed (and unsigned) checksums to the github release
page. Once the current commit is tagged as a release, run `make release`
to build the packages and upload them to github.

.gitignore

@@ -13,4 +13,4 @@ mgmt.static
 build/mgmt-*
 mgmt.iml
 rpmbuild/
-*.deb
+releases/