user detection in `install` is busted on some Linux systems

A user reported in IRC that his `id` command outputs the following:

UID=1000(mitch) GID=1000(mitch) groepen=1000(mitch),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),121(sambashare)

...which is not handled by the regular expression we're using in `install`. I don't know why we're not using `id -u -n` but we should be. As far as I know it works everywhere we support OpenNMS.