MFC svn 267065 and 267187

make sure ifp->if_transmit returns 0 if a buffer is enqueued.
This should also be merged to stable/9.

After this fix, drivers still known to have this bug are igxbe/ixv
and i40e.

Drivers using if_transmit are correct, and so are most of the
other drivers that reassing if_transmit.

Among other things, this bug causes panics when using netmap emulation
on top of generic drivers.
MFC 267167: whitespace changes (comments)
MFC 267168:
misc bugfixes:
- stdio.h is needed for fprint()
- make memsize uint32_t to avoid errors due to overflow
- honor the *XPOLL flagg in NIOCREGIF requests
- mmap fails wit MAP_FAILED, not NULL.