Add -l (--log-level) flag to override default BPF verifier log lever.
This only matters in verbose mode, which is the mode in which veristat
emits verifier log for each processed BPF program.

This is important because for successfully verified BPF programs
log_level 1 is empty, as BPF verifier truncates all the successfully
verified paths. So -l2 is the only way to actually get BPF verifier log
in practice. It looks sometihng like this:

  [vmuser@archvm bpf]$ sudo ./veristat xdp_tx.bpf.o -vl2
  Processing 'xdp_tx.bpf.o'...
  PROCESSING xdp_tx.bpf.o/xdp_tx, DURATION US: 19, VERDICT: success, VERIFIER LOG:
  func#0 @0
  0: R1=ctx(off=0,imm=0) R10=fp0
  ; return XDP_TX;
  0: (b4) w0 = 3                        ; R0_w=3
  1: (95) exit
  verification time 19 usec
  stack depth 0
  processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

  File          Program  Verdict  Duration (us)  Total insns  Total states  Peak states
  ------------  -------  -------  -------------  -----------  ------------  -----------
  xdp_tx.bpf.o  xdp_tx   success             19            2             0            0
  ------------  -------  -------  -------------  -----------  ------------  -----------
  Done. Processed 1 files, 0 programs. Skipped 1 files, 0 programs.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-6-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>