libnm: merge libnm-vpn into libnm

libnm: merge libnm-vpn into libnm

There's not much point in keeping them separate: all existing
libnm-glib-vpn users also link against libnm-glib, and the amount of
extra code added to libnm by merging in libnm-vpn is negligible.

Additionally, nm-vpn-plugin will later need access to some
libnm-internal APIs.

So, merge them together.