Align naming and commenting of mechanism for receiving oversize packets across protocols

Align naming and commenting of mechanism for receiving oversize packets across protocols

We've now implemented mechanisms to tolerate larger-than-expected packets for:

  - Uncompressed CSTP packets ("Fixed regression with CSTP MTU handling"
    patch in July 2016)

  - Uncompressed oNCP packets ("Do not drop vpn connection if packet arrived
    is larger than MTU" patch in May 2017)

  - Uncompressed GPST packets (in original merge from March 2018; this is a
    virtual necessity for GlobalProtect because it has no functional
    mechanism for negotiating the MTU)

  - Uncompressed ESP packets ("check for oversize ESP packets, with 256
    bytes of headroom above calculated" in March 2018; GlobalProtect requires
    this for the aforementioned reason)

  - Compressed CSTP packets (preceding patch in this series)

Since this is a requiring issue across protocols, it's useful to align the
naming, commenting, and packet sizing-tolerance across the source files.

  1) Use receive_mtu everywhere as the name for the maximum tolerated size of an
     incoming packet.
  2) Insert similar comments explaining its purpose everywhere it's used.
  3) Use receive_mtu = MAX(16384, vpninfo->ip_info.mtu) for all TLS-based
     tunnels, because 16384 is the maximum TLS record size.
  4) Use receive_mtu = MAX(2048, vpninfo->vpninfo->ip_info.mtu + 256) for
     all UDP-based tunnels, because the MTU of IP datagrams on the public
     internet is effectively ~1500.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>