Increase the error buffer size for rpcap.

If the rpcap port is unreachable pcap_findalldevs_ex can write more
than PCAP_ERRBUF_SIZE bytes to errbuf. E.g. if we try to capture from
Google's all-eights public DNS server we get:

----
Can't get list of interfaces: Is the server properly installed on 8.8.8.8?
connect() failed: A connection attempt failed because the connected
party did not properly respond after a period of time, or established
connection failed because connected host has failed to respond.  (code 1
----

Set the buffer to PCAP_ERRBUF_SIZE*4 bytes. Hopefully that's large enough.

Change-Id: I19f34cda16050c1ba8b9d7d6ed2d8e77b945a2af
Ping-Bug: 3554
Ping-Bug: 6922
Ping-Bug: 7021
Reviewed-on: https://code.wireshark.org/review/3880
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>