lib.pcap: Fix memory corruption bug affecting unit tests

The bug could cause occasional segmentation faults in code that uses
the pcap reader library, such as the ipv6 and packet filter unit tests.

The bug is that ffi.copy() writes one by more than we want. See caveat
mentioned in the LuaJIT docs at http://luajit.org/ext_ffi_api.html:

    ffi.copy(dst, src):
    All bytes of the string plus a zero-terminator are copied to
    dst (i.e. #src+1 bytes).

Amusingly, this bug was introduced by the previous change (commit
8def5910) that simultaneously fixed a different more-or-less
equivalent bug in the same function.
lib.pcap: Remove 'extra' nonstandard pcap format extension

This was an old hack to store additional metadata on pcap records. Wed
don't use it anymore.
pci.lua: Scan devices in selftest() and remove dead code