Enable and ship the su binary in util-linux package

su pam config from src:shadow, su-l from fedora util-linux package.

TODO: a/ consider shipping/taking /etc/login.defs (from src:shadow)
      or patch hardcoded defaults to match debian expectiations:
       _PATH_DEFPATH to include /usr/local/games:/usr/games
       TTYPERM 0600
       SYS_UID_MIN 100
       CHFN_RESTRICT rwh (== yes)
      b/ add versioned Replaces/Breaks against login package.
      c/ add util-linux.NEWS entry about using util-linux su
         and which relevant login.defs settings (only SYSLOG_SU_ENAB?)
         are now ignored.