Sampling use SIGRTMIN + N signals (#104)

* Use SIGRTMIN instead of SIGALRM for sampling

* Config options + fully working SIGRTMIN sampling

- OMNITRACE_SAMPLING_KEEP_INTERNAL config option
- OMNITRACE_PROCESS_SAMPLING_FREQ config option
- OMNITRACE_SAMPLING_REALTIME config option
- OMNITRACE_SAMPLING_CPUTIME config option
- OMNITRACE_SAMPLING_REALTIME_OFFSET config option

* Fix omnitrace-avail-regex-negation test

- OMNITRACE_PROCESS_SAMPLING_FREQ was causing failure

source/bin/tests/CMakeLists.txt

@@ -252,6 +252,7 @@ omnitrace_add_bin_test(
          -r
          _P
          ~PERFETTO
+         ~PROCESS_SAMPLING
          --csv
          --brief
     LABELS "omnitrace-avail"