Also simplified the argument parsing to write all currently supported
CLI options into a bash array and pass this to `nix-build`.
Also documented `--option` usage in the corresponding manpage.
- `localSystem` is added, it strictly supercedes system
- `crossSystem`'s description mentions `localSystem` (and vice versa).
- No more weird special casing I don't even understand
TEMP
* manual: Mark commands that require root
Mark every command that requires to be run as root by prefixing them
with '#' instead of '$'.
* manual: Add note about commands that require root
