Have the routines to get interface lists take a pointer to a "gchar *"

Have the routines to get interface lists take a pointer to a "gchar *"
as an argument, and, on an error, if they have an error message, have
them set that "gchar *" to point to a g_malloc()ed string containing the
error message, rather than taking a pointer to a buffer for that message
as an argument.  That's more like what's done in Wiretap, and doesn't
impose an upper limit on the lengths of those error messages.  If that
pointer is null, don't allocate the message string and return it.

Have that error message already have the "cant_get" processing applied
to it, so nobody other than those routines need to call the "cant_get"
routines to process the error messages.

Have get_airpcap_interface_list() explicitly set "*err" to the
appropriate error code.

Clean up indentation.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@20521 f5534014-38df-0310-8fa8-9805f1628bb7