kernel/netgraph7: Sync ng_bt3c with current FreeBSD and port it over.

I can't test this hardware unfortunately since I don't own it.

Netgraph port from FreeBSD - initial porting work

* Use relative #include's as appropriate so the code doesn't
dig into the original netgraph/ directory.

* Move FBSDID's and add DragonFly ids.

*

Netgraph port from FreeBSD - initial porting work

* Use relative #include's as appropriate so the code doesn't
dig into the original netgraph/ directory.

* Move FBSDID's and add DragonFly ids.

* Fix improper uses of M_NOWAIT.

* Fix uses of M_NOWAIT which should be MB_DONTWAIT and vise-versa
(also forward to Julian).

* Add a dragonfly.h compatibility header.

* NOTE: m_tag_alloc() in DFly takes MB_ flags.

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Vendor import of netgraph from FreeBSD-current 20080626