dahdi: Restore DAHDI_CONFLINK functionality as compile time option.

This is mostly a revert of commit r9463. If you need to use DAHDI_CONFLINK
ioctl, make sure to define CONFIG_DAHDI_CONFLINK in
include/dahdi/dahdi_config.h. Apparently there were some users of CONFLINK out
there still.

It's a compile time option now since most users won't need to run the test for
conflinks in the hot-path that is the process_masterspan function.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Tested-by: Ted Gerold <ted@twg.org>
dahdi: Tear down conference links when conferences are emptied out.

Otherwise, it's possible for a link to remain in use if a process quits without
unlinking the conferences. Now when all the channels are removed from the
conference, any links are also cleaned up.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>