Add MPTCP support with the --multipath flag

[marcosfsch]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any):
  Rebase of PR #1166, including testing for MPTCP support.

• Brief description of code changes (suitable for use as a commit message):
  Recent version of the Linux kernel (5.9) gained MPTCP support, this change
  allows easy testing such protocol.
  MPTCP is strongly tied to TCP, and the kernel APIs are almost the same,
  so this change does not implement a new 'struct protocol'. Instead it just
  extend the TCP support to optionally enable multipath via the --multipath or
  -m switch.

The only required dependency is the 'IPPROTO_MPTCP' protocol number
definition, which should be provided by the netinet/in.h header.
To keep things simple, just conditionally provide the required
protocol, if the system header does not have it yet.