make ipfw_check_packet() and ipfw_check_frame() public,
so they can be used in the userspace version of ipfw/dummynet
(normally using netmap for the I/O path).

This is the first of a few commits to ease compiling the
ipfw kernel code in userspace.
more support for userspace compiling of this code:
emulate the uma_zone for dynamic rules.
make this code compile in userspace on OSX
disable some ipfw match options when compiling in userspace
add a counter on the struct mq (a queue of mbufs),
and add a block for userspace compiling.