Support socket activation

There are now separate properties for the port that is "displayed" (e.g.
when issuing redirects) and the port that is actually bound. The bind
ports may also be set to "stdin", in which case System.inheritedChannel
is used (expects a socket as file descriptor 0). This allows gigi to
inherit a socket from the system manager ((x)inetd, systemd), which in
turn allows one to run gigi as any user on root ports (e.g. port 80).

Change-Id: I343e1e25daae94aae67db1dd6f25fcfb6241d0fc