[mapplauncherd] Drop user/group ID only if not privileged

Previously, the user ID and group ID of the process was always set
to that of the application, if it differed from that of the invoker.
This commit adds simple privileges checking for the application
to determine whether or not it is allowed to access privileged
system information, before deciding to drop user ID or group ID.
Merge pull request #10 from chriadam/privs

[mapplauncherd] Drop user/group ID only if not privileged