trace: allow enabling packet trace at runtime

[rjarry]

If the -t, --test-mode flag is used, enable packet tracing instead of relying on an undocumented compilation flag. The performance impact should be minimal when it is not enabled.

[david-marchand]

On the principle, it looks good, but I suspect it has a non negligeable impact on performance.

[rjarry]

Out of curiosity, I ran a simple IP forwarding performance test, before and after this patch.

before: 23.56 MPPS

GROUT: main: starting grout version v0.1-1-gbed15f4ef48e

NODE        CALLS  PACKETS  PKTS/CALL  CYCLES/CALL  CYCLES/PKT
ip_input     1988   315693      158.8       8367.9        52.7
port_rx      2176   315693      145.1       5573.3        38.4
port_tx      1988   315693      158.8       4400.9        27.7
eth_input    2176   315693      145.1       3263.6        22.5
ip_output    1988   315693      158.8       3168.4        20.0
eth_output   1988   315693      158.8       3135.6        19.7
ip_forward   1988   315693      158.8       1741.1        11.0

after: 23.56 MPPS

GROUT: main: starting grout version v0.1-2-ge3133c7c1c80

NODE        CALLS  PACKETS  PKTS/CALL  CYCLES/CALL  CYCLES/PKT
ip_input     2354   315196      133.9       7055.5        52.7
port_rx      2624   315196      120.1       4626.7        38.5
port_tx      2354   315196      133.9       3678.2        27.5
eth_input    2624   315196      120.1       2708.8        22.6
ip_output    2354   315196      133.9       2681.7        20.0
eth_output   2354   315196      133.9       2621.4        19.6
ip_forward   2354   315196      133.9       1455.5        10.9

If this patch has any impact, it is not noticeable in this particular test.