Bump to sci-libs/comedilib-9999 and add sci-misc/comedi-headers-9999.

Bump to sci-libs/comedilib-9999 and add sci-misc/comedi-headers-9999.

The tricky part is handling /usr/include/comedi.h, which is supplied
by:

* the kernel's staging comedi drivers (but not installed)
    drivers/staging/comedi/comedi.h
* the Comedi module (installed via sci-misc/comedi-headers):
    include/linux/comedi.h
* the Comedilib module (not installed)
    include/comedi.h

The current CSV sources are very similar, but there are reasonable
differences with the staging version.  Hopefully everything works out
;).

The dependency setup is based on media-libs/alsa-lib and
media-sound/alsa-headers, which have to deal with a similar setup (in
kernal driver, out of kernal driver, and userspace library).