DEBUG: opentracing: show return values of all functions in the debug output

DEBUG: opentracing: show return values of all functions in the debug output

If the OpenTracing filter is compiled using the 'OT_DEBUG=1' option, then
log messages are printed to stderr when the filter is running.  In the log
one can then find (among other things) the order in which the function is
called and the value that the function returns (if it is not a void type).

Prior to applying this patch, no value returned by a function was logged.

Log output example:
  [ 1]    0.038807 [OT]: flt_ot_init_per_thread(0x56365bd45ec0, 0x56365bd48210) {
  [ 1]    0.038807 [OT]:    ot_start(0x56365bd58920, 0x56365bd4e3a0, 0x7f561acba168:(nil)) {
  [ 1]    0.038807 [OT]:    } = 0
  [ 1]    0.038807 [OT]: } = 0

This patch must be backported as far as 2.4.