If has_snaplen isn't set, don't set the snapshot length with pcap_create()/pcap_activate().
Just let libpcap pick the snapshot length; that way, for link-layer
types that need a really large snapshot length, such as D-Bus (which
requires 128MB for the largest messages), it can pick that, but can
otherwise pick something that doesn't require as much memory, e.g.
256KB.
For pcap_open_live() and pcap_open(), which don't have a way of saying
"give me what's appropriate", pick 256KB.
Change-Id: Idef5694f7dfa85eaf3a61d6ca7a17d263c417431
Reviewed-on: https://code.wireshark.org/review/21917
Reviewed-by: Guy Harris <guy@alum.mit.edu>