Fix bug http://bugs.wireshark.org/bugzilla/show_bug.cgi?id=1757 :

Fix bug bugs.wireshark.org/bugzilla/show_bug.cgi?id=1757 :

Try to call get_interface_descriptive_name() as little as possible (storing
the result in capture_opts) to avoid a performance hit during live capture
(especially if you have lots of interfaces) and to avoid leaking memory.

One issue with this is that capture_opts.c cannot (without adding significant
dependencies) set the iface_descr so readers of that field (only gtk/main.c
and tshark.c) use a macro to (set if not already set and) get the value of
that field.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@22587 f5534014-38df-0310-8fa8-9805f1628bb7