add features to properly support using a shared install target with multiple users

in the end, this should add support for:

* [x] `--umask`: set custom umask (and don't override it)
* [x] `--group`: set group to be used in installations (only verify) 
* [x] `--set-gid-bit`: set group ID bit on directories (to make group ownership trickle down to new dirs/files)
* [x] `--sticky-bit`: set sticky bit on directories (to avoid that one user destroys the work of another user)