vmci(4): Fix GCC build and rationalize vmci_kernel_defs.h

To fix the GCC build, remove multiple redundant declarations of
vmci_send_datagram() (the copy in vmci.h as well as the extern definition in
vmci_queue_pair.c were wholly redundant).

Also to fix the GCC build, include a non-empty format string in the vmci(4)
definition of ASSERT(). It seems harmless either way, but adding the
stringified invariant is easier than masking the warning.

The other vmci_kernel_defs.h changes are cosmetic and simply match macros to
existing definitions.

Reported by: GCC 6.4.0
Sponsored by: Dell EMC Isilon