Having a command to test BPF programs

Having a command to test BPF programs would ease the development of new BPF programs for Gatekeeper since this command can
1. Verify that both functions (i.e. "init" and "pkt") are present;
2. Check that all symbols can be resolved; and
3. Make sure that both functions pass the BPF verifier.

The code of this command may be able to reuse some of the code in file `gk/bpf.c`.

The name suggestion for this command is "gkbpftest".