Renamed unrpm to original rpmunpack, so you can use an included shell script
called unrpm as a front end to it.
There's also a shell script called undeb included for debian packages.
Change undeb and unrpm scripts to work with the busybox tar and gunzip.
a few other ugly places (do_subst_command got a much-needed overhaul). Also
took out BB_FEATURE_SED_PATTERN_SPACE from Config.h[.Hurd] as the 'p' is now a
standard feature (adds almost no bloat).
