Add O_EXTERNAL_DATA opcode support.

This opcode can be used to attach some data to external action opcode.
And unlike to O_EXTERNAL_INSTANCE opcode, this opcode does not require
creating of named instance to pass configuration arguments to external
action handler. The data is coming just next to O_EXTERNAL_ACTION opcode.

The userlevel part currenly supports formatting for opcode with ipfw_insn
size, by default it expects u16 numeric value in the arg1.

Obtained from: Yandex LLC
MFC after: 2 weeks
Sponsored by: Yandex LLC