implement ethernet over gre encapsulation with a thing egre(4) driver.

implement ethernet over gre encapsulation with a thing egre(4) driver.

there's three main chunks in this: transmit, receive, and factoring out
common gre code.

ethernet over gre unfortunately doesnt pad ethernet frames, so their
payload on rx and their headers on tx will be misaligned. egre copes
with this in the rx path by copying the payload with m_dup_pkt if
it detects misalignment. in the tx path, it unconditionally allocates
a new mbuf for the headers, that can be aligned separately to the
existing payload.

most of the common gre code is in the ioctl paths, so this diff
adds gre_tunnel_ioctl, which is in the spirit of ether_ioctl.

ok claudio@