(issue54) Implement a privilege drop to execute the program after installation. This commit is extremly ugly as I accidentally worked in a working tree that was partially merged with default. To review the real change please check the commit that will merge this branch into default.

aheinecke@intevation.de=Andre Heinecke
andre.heinecke@intevation.de=Andre Heinecke
bernhard@intevation.de=Bernhard Reiter
rrenkert@intevation.de=Raimund Renkert
wilde@intevation.de=Sascha Wilde
emanuel@intevation.de=Emanuel Schütze