selftests: mptcp: add command line arguments for mptcp_join.sh

Since the mptcp_join script is becoming too big, this patch splits it
into several smaller chunks, each of them has been defined in a function
as a individual test group for several related testcases.

Using bash getopts function to parse command line arguments, and invoke
each function to do the individual test group.

Here are all the arguments:
  -f subflows_tests
  -s signal_address_tests
  -l link_failure_tests
  -t add_addr_timeout_tests
  -r remove_tests
  -a add_tests
  -6 ipv6_tests
  -4 v4mapped_tests
  -b backup_tests
  -p add_addr_ports_tests
  -c syncookies_tests
  -h help

Run mptcp_join.sh with no argument will execute all testcases.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>