if_gif(4): Support the IFF_LINK0 flag to change the MTU handling for IPv6

[hrs]

Summary

The gif(4) interface always used IPV6_MINMTU when the outer protocol was
IPv6 so that packets would not be dropped between the tunnel endpoints.
The interface MTU was ignored because path MTU discovery was too expensive.

This change adds IFF_LINK0 flag support to revert it to normal behavior,
and documents the backgrounds. While using IPV6_MINMTU is still
the safest option for IPv6, ignoring the configured interface MTU is
confuses the users, and using a larger MTU is sometimes useful.

[ae]

Maybe it would be better implement such feature via named flag, like ignore_source is implemented? Also if_gre(4) has the same problem.

[bcr]

OK for the man page part of the change.