Replace "whoami" call with call to "id" in install_server.sh

`id -u` is in the POSIX and LSB specs [1], while `whoami` is not.
Furthermore, the username of UID 0 can be changed (and is sometimes
recommended in a misguided attempt to increase security).

FreeBSD users logged in as `toor` would also slip through the cracks.

[1]: http://pubs.opengroup.org/onlinepubs/009695399/utilities/id.html
     http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic/command.html