vss_send(): Fix EOF-check for FEC clients.

vss_send(): Fix EOF-check for FEC clients.

If the last chunk has been sent to all http/dccp clients we have
to wait until the last FEC group has been sent before setting the
NEXT flag that causes all senders to shut down its clients. The old
code tested if a slice was sent to any FEC  client during vss_send()
and set the NEXT flag if nothing was sent.

However, this is not sufficient as there may be still slices available
which have to be sent at some future time. This patch teaches
vss_send() to detect this condition. It also renames the boolean
variable sent_something to fec_active, which is more to the point.