MINOR: trace: add the long awaited TRACE_PRINTF()

MINOR: trace: add the long awaited TRACE_PRINTF()

TRACE_PRINTF() can be used to produce arbitrary trace contents at any
trace level. It uses the exact same arguments as other TRACE_* macros,
but here they are mandatory since they are followed by the format-string,
though they may be filled with zeroes. The reason for the arguments is to
match tracking or filtering and not pollute other non-inspected objects.

It will probably be used inside loops, in which case there are two points
to be careful about:
  - output atomicity is only per-message, so competing threads may see
    their messages interleaved. As such, it is recommended that the
    caller places a recognizable unique context at the beginning of the
    message such as a connection pointer.
  - iterating over arrays or lists for all requests could be very
    expensive. In order to avoid this it is best to condition the call
    via TRACE_ENABLED() with the same arguments, which will return the
    same decision.
  - messages longer than TRACE_MAX_MSG-1 (1023 by default) will be
    truncated.

For example, in order to dump the list of HTTP headers between hpack
and h2:

  if (outlen > 0 &&
      TRACE_ENABLED(TRACE_LEVEL_DEVELOPER,
      H2_EV_RX_FRAME|H2_EV_RX_HDR, h2c->conn, 0, 0, 0)) {
          int i;
          for (i = 0; list[i].n.len; i++)
              TRACE_PRINTF(TRACE_LEVEL_DEVELOPER, H2_EV_RX_FRAME|H2_EV_RX_HDR,
                           h2c->conn, 0, 0, 0, "h2c=%p hdr[%d]=%s:%s", h2c, i,
                           list[i].n.ptr, list[i].v.ptr);
  }

In addition, a lower-level TRACE_PRINTF_LOC() macro is provided, that takes
two extra arguments, the caller's location and the caller's function name.
This will allow to emit composite traces from central functions on the
behalf of another one.