core: Remove dead symbols from asterisk.exports.in.

* dahdi_chan_name
* dahdi_chan_name_len
* dahdi_chan_mode
* __manager_event
* dialed_interface_info

Added comment about __progname and environ being needed for FreeBSD to
prevent accidental removal in the future.

Change-Id: I3ae026bc541cd9cb572be2ffa95fc359547642b5