socket: properly support SO_ATTACH_FILTER sockopt

[jow-]

The SO_ATTACH_FILTER socket option requires special handling as it is not passing a self-contained structure to the kernel but a pointer to user memory holding the actual BPF bytecode.

In order to properly support this, first rework the ucode value to C struct conversion callback machinery to pass an indirect struct base pointer, allowing conversion callbacks to realloc the struct memory as needed.

Finally introduce custom uv to C conversion for the BPF data which accepts either a raw bytecode string, an array of opcode arrays or a flat array of opcode values which are converted into a C array of struct sock_filter records appended to the reallocated struct memory.

Attaching a BPF program equivalent to the tcpdump expression vlan 20 && ether proto 0x1234 would then look like this:

sock.setopt(SOL_SOCKET, SO_ATTACH_FILTER, {
  filter: [
    [ 0x28, 0, 0, 0x0000000c ],
    [ 0x15, 2, 0, 0x00008100 ],
    [ 0x15, 1, 0, 0x000088a8 ],
    [ 0x15, 0, 6, 0x00009100 ],
    [ 0x28, 0, 0, 0x0000000e ],
    [ 0x54, 0, 0, 0x00000fff ],
    [ 0x15, 0, 3, 0x00000014 ],
    [ 0x28, 0, 0, 0x00000010 ],
    [ 0x15, 0, 1, 0x00001234 ],
    [ 0x6, 0, 0, 0x00040000 ],
    [ 0x6, 0, 0, 0x00000000 ],
  ]
});