Remove signing support from (lib)rpmbuild

Remove signing support from (lib)rpmbuild
- Signing is better done in an environment completely separated from
  the process of actually building packages. This is how its generally
  done anyway by distros, this change just forces the issue.
- While signing on build seems handy and saves some io-churn, this
  is very insecure as your precious secret passphrase remains unencrypted
  in memory for the entire duration of the build where it's accessible
  to whatever happens to be building. This change doesn't
  affect fix the issue of unencrypted passphrase residing in memory
  and potentially getting swapped out, it only limits the exposure
  to the actual signing process which isn't subject to unknown scripts
  poking around.
- Minimally preserve rpmbuild's --sign functionality via popt --pipe hack
  to call rpm --addsign when build completes