GLib/Gtk have mostly settled on the convention that two-letter
acronyms in type names remain all-caps (eg, "IO"), but longer acronyms
become initial-caps-only (eg, "Tcp").

NM was inconsistent, with most long acronyms using initial caps only
(Adsl, Cdma, Dcb, Gsm, Olpc, Vlan), but others using all caps (DHCP,
PPP, PPPOE, VPN). Fix libnm and src/ to use initial-caps only for all
three-or-more-letter-long acronyms (and update nmcli and nmtui for the
libnm changes).