syscall.9: Add a rough explanation of how a syscall ends up in the kernel.
netmap: change header includes

Suggested-by: sephe

FWIW, kqfilter() port is not really going well as poll() is doing
a bunch of loosely related drive-by actions that somehow form the
API.  The best approach would be to thin out the implementation to
at least get the bare core functionality working.  That'll also
include writing new userland glue.  Feels more like a fork than a
port that way.  :(
netmap: properly store private device data

Yes, one could go through the trouble of redefining the cdevpriv
functions as macros returning wild errors, but it looks a lot
cleaner in native code.
kernel/ath: Make it compile into the kernel again and re-add to GENERIC.

Full list of changes:

* Adjust the 'files' file for changed ath_hal path and additional
  source.

* Add options from FreeBSD and comment those out which are not
  compiling/used. Add the remaining ones to the LINT kernels for
  additional build checking.

* Fix some typos in the ar9300 code.

* Use ath_hal_ether_sprintf() where needed.

* Add some casts to work around -Wenum-compare and -Wformat breakage.

* Comment out some unused struct ifp's and FreeBSD's ktr.

* Slightly readjust ATH_RXBUF and ATH_TXBUF handling to let the kernel
  options of the same name override the defaults. In the GENERIC kernel
  and module everything stays the same.

* Use the (slightly better structured) FreeBSD Makefile for ath_hal.

All kernels compile with ath(4) as of this commit.
netmap: reduce upstream diff output; bring back __FBSDIDs
Update ath.4 and ath_hal.4 manual pages from FreeBSD.