pxpcap: Allow capturing to bytearray

Capturing used to always be to str/bytes, but can now be set to bytearray.
pxpcap: Set arbitrary properties via constructor

This is experimental.  The idea is that a user can set arbitrary
metadata on the pcap object before capture begins without having
to explictily call start().
pcap_switch: Start capturing later

We now set up all interfaces and do some other initialization
before starting capture.  This may avoid races.