selftests: mptcp: export ip_mptcp to mptcp_lib

This patch exports ip_mptcp into mptcp_lib.sh as a public variable,
named MPTCP_LIB_IP_MPTCP. Add a helper mptcp_lib_set_ip_mptcp() to set
it, and a helper mptcp_lib_is_ip_mptcp() to test whether it is set. Use
these two helpers in mptcp_join.sh.

This patch is prepared for coming commits.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>