[media] DocBook/dvb: Use generic descriptions for the frontend API

Move generic stuff into gen-errors.xml, and remove them from
DVB API. While here, removes two bogus error codes that aren't
supported or used on Linux: EINTERNAL and ENOSIGNAL.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>