Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Clean up DDP layer netatalk code:

- General style(9) cleanup -- white space, braces, line wraps, etc.

- Annotate a lack of synchronization the global route cache if the input
routine is invoked with parallelism.

- Remove unused debugging code.

Break out my copyrights with our 2-clause BSD license from the UMich
copyright, which while similar, is different.

MFC after: 3 days

/* -> /*- for license, minor formatting changes, insert COPYRIGHT into files

Procotol control block locking for netatalk DDP.

Isolate PCB-specific ethertalk DDP functions in ddp_pcb.c, removing them
from ddp_usrreq.c. Functions moved are:

at_pcballoc()
at_pcbconnect()
at_pcbdetach()
at_pcbdisconnect()
at_pcbsetaddr()
at_sockaddr()

Also moved are ddp_ports and ddpcb, global variables associated with DDP
pcbs. This makes PCB implementation more parallel to inet, inet6, and
ipx.