Disable automatic MTU adjustment for if_gre(4) and if_me(4)

[ae]

Summary

In the rS274246 the behavior of MTU calculation was partially changed.
Before this revision interface MTU was adjusted only when GRE key is configured. In the rS274246 when some option, that changes the size of GRE header is configured, the MTU value automatically recalculated.
Several users have complained that this is not handy, and better when an administrator does all calculations and if he wants MTU 1500, it should be 1500 after ifconfig gre0 mtu 1500.
This patch removes automatic MTU adjustment from if_gre(4) and if_me(4).