Add support for MAP-E FMRs (mesh mode)

MAP-E FMRs (draft-ietf-softwire-map-10) are rules for IPv4-communication
between MAP CEs (mesh mode) without the need to forward such data to a
border relay. This is similar to how 6rd works but for IPv4 over IPv6.

Signed-off-by: Steven Barth <cyrus@openwrt.org>