TODO:

- The temporary directory name "blubber" should be replaced by a
  random dynamic name, similar to the "mktemp" command.

- rpmbuild insists to use "./configure". It would be good
  to change this to eliminate then the dummy "configure" script.

- To be able to include the resulting shellscript into a spec file, it
  might be necessary to modify it, to not disturb the spec parser of
  rpmbuild. There are two possibilities: sourcing it, or using a (not
  very neat) "ASCII blob", which will not interfere with the syntax
  necessities of spec files. Perl has uuencoding built in, without
  any module dependencies (see "perldoc -f pack").
  So the ASCII blob could be de/encoded by piping it through Perl.

 - Fail installation process if user exists already (useradd returns 9),
   because when package is removed, the users home directory will be deleted.

 - Set expire-date for user (configurable).