nolimits: Require that wrapped commands are absolute paths.

Otherwise, users could exploit this by setting $PATH and renaming
executables to execute any payload without resource limits.
nolimits: Add licence text and program description.
nolimits.conf: Add relative pathnames as well.

When the shell finds the command in $PATH, only the search term is put
into argv[0], not the full pathname.
nolimits.conf: Remove `rsyncd` from the allowed commands.

Due to its `pre-xfer exec` and `post-xfer exec` settings,
it could be used to execute arbitrary payload.