net/mpd5: minor improvements
Import several improvement from upstream:
- r2408,2409 - use SOCK_CLOEXEC for all sockets including PPtP, web and telnet consoles, so that subprocesses like ip-up/down scripts do not inherit such sockets;
- r2415 - avoid inadequate MTU undervalueing when bundle compression and/or encryption configured but not negotiated; f.e. PPPoE interface MTU could be set to 1490 instead of 1492 previously;
- r2482,2483 - improve client-side implementation of PPP-Max-Payload (RFC4638) and do not limit "set pppoe max-payload {size}" to 1510 but use MTU of parent interface minus 8, as per RFC; also, relax sanity check for "set link mtu/mru" in case of PPPoE, so that it is possible to configure values like 1500 and over, if needed.