start*/enter-chroot: Make sure options with missing arguments are detected

Mark the end of user specified parameters with `""`.

Makes sure commands like `sudo startcli -u` fail instead of outputing confusing messages like `user exec not found`).

Tested in `2014-03-03_13-05-54_drinkcat_chroagh_fix-startXX-arg_-R_precise_1`.

--

For reference, there is a neater way to detect empty arguments:
```
if [ -z "$OPTARG" -a "${OPTARG-unset}" != "unset" ]; then
    error 2 "$USAGE"
fi
```
but `dash` is not POSIX-compliant in this case, and does not unset `OPTARG`...