Return warning indications from the activate routine.

Initialize the status variable to "no error or warning" (0); set it for
all error or warning conditions.  Return that variable if we haven't
failed.
Don't discard warnings.

If we've set status to a PCAP_WARNING_ value, don't use it to store the
return value of any function afterwards.
Add LINKTYPE_EPON/DLT_EPON.

Also add some missing elements to the dlt_choices[] table, and move the
entry for DLT_DBUS where it belongs (in order by type).
In activate routines, don't set status unless a warning or error occurs.

This makes the code a bit clearer, as you don't have to worry about the
status variable accidentally being set to an inappropriate value,
overriding a warning that's been put there.
In activate routines, don't set status unless a warning or error occurs.

This makes the code a bit clearer, as you don't have to worry about the
status variable accidentally being set to an inappropriate value,
overriding a warning that's been put there.