core: Move NMPlatformSource to nm-types.h

...and rename it while at it. It's going to be useful outside nm-platform,
to weight MTU options from various sources.

src/dhcp-manager/nm-dhcp-dhclient-utils.c

@@ -623,7 +623,7 @@ nm_dhcp_dhclient_read_lease_ip_configs (const char *iface,
 
 		address.timestamp = now_monotonic_ts;
 		address.lifetime = address.preferred = expiry;
-		address.source = NM_PLATFORM_SOURCE_DHCP;
+		address.source = NM_IP_CONFIG_SOURCE_DHCP;
 
 		ip4 = nm_ip4_config_new ();
 		nm_ip4_config_add_address (ip4, &address);