vnode_fd and kqfd are both shared among multiple CU; define them exactly
once.
In the case of vnode_fd, it was simply the declaration that needed
correction.
-fno-common will become the default in GCC10/LLVM11.
MFC after: 3 days
This is being done to clearly distinguish the libkqueue tests
from the (soon to be imported) NetBSD tests.
MFC after: 58 days
Sponsored by: EMC / Isilon Storage Division
