* Add multipath TCP (MPTCP) support. NetworkManager can now configure IP addresses
  as MPTCP endpoints. This is configurable via the "connection.mptcp-flags"
  property. The default setting is such that MPTCP handling is automatically
  enabled if the kernel sysctl "/proc/sys/net/mptcp/enabled" indicates so.
  NetworkManager does not enable the MPTCP sysctl or adjust the limits (ip mptcp limits).
  The administrator or the distribution is supposed to configure the desired system
  settings.
  Note that strict reverse path filtering (rp_filter) breaks many MPTCP use cases.
  With MPTCP handling enabled, NetworkManager will relax a strict (1) rp_filter
  to loose (2). Otherwise rp_filter is untouched by NetworkManager.