Don't close the currently loaded capture file in capture_start(), do so

Don't close the currently loaded capture file in capture_start(), do so
in its callers; when starting a capture with "wireshark -k", there's no
capture file to close, and closing it might be provoking some UI actions
that cause crashes on Windows.

Don't copy the list of selected interfaces to the list of capture
interfaces in capture_start(), either, do that in the callers; we were
already doing that in one place and, in one of the remaining cases,
namely "wireshark -k", we should do so only if no capture interfaces
were supplied on the command line.  (I.e., the set of interfaces on
which we want to capture depends on where in the UI the capture is being
started.)

svn path=/trunk/; revision=41491