mgd: emit a portfile to make testing multiple daemons on the same machine easier

[rcgoodfellow]

The omicron testing strategy involves running a large pile of mgd daemons at once. These daemons all listen on different ports, and the omicron test code sifts through mgd log files to determine the port. Things would be much simpler if mgd just wrote out a portfile with the listening port number in it.