mptcp: selftests: add rcvbuf set option

allows to run the tests with fixed receive buffer by passing
"-R <value>" to mptcp_connect.sh.

While at it, add a default 10 second poll timeout so the "-t"
becomes optional -- this makes mptcp_connect simpler to use
during manual testing.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>