setkey: MFC: load ipsec.ko automatically
If setkey(8) is used without ipsec.ko loaded beforehand,
its attempt to install SA/SPD into the kernel results in cryptic
EINVAL error code.
Let it be a bit more user-friendly and try to load ipsec.ko
automatically if it is not loaded, just like ifconfig(8) does it
for modules it needs.
(cherry picked from commit 0aef8628458a7d03e3c7e63ae05e228191167eec)