This allow retrieving the value of the known PMTU for the current
socket. This works on Linux (IPv6 and IPv4) and FreeBSD (IPv6 only) --
the other OSes don't have the necessary API.
Note: do we need add IP_MTU_DISCOVER?
Change-Id: I6e9274c1e7444ad48c81fffd14dcaf97a18ce335
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>