introduce `get_user_shell()`
`get_user_shell()` to distinguish root and $CVMFS_USER
replace occurences of sudo -u $CVMFS_USER [...]
remove superfluid `sh -c` calls
documentation of `get_user_shell()`
Merge pull request #581 from reneme/feature-centralise_sudo_use

Feature: Centralise the Use of `sudo` in `cvmfs_server`