Fix LD_PRELOAD (#184)

- __libc_start_main in libomnitrace-dl wasn't wrapping bc of -fvisibility=hidden
- fix OMNITRACE_STRIP_TARGET
- omnitrace_reset_preload function in main library
- defer removing libomnitrace from LD_PRELOAD

source/lib/omnitrace/api.cpp

@@ -90,6 +90,12 @@ omnitrace_finalize(void)
     omnitrace_finalize_hidden();
 }
 
+extern "C" void
+omnitrace_reset_preload(void)
+{
+    omnitrace_reset_preload_hidden();
+}
+
 extern "C" void
 omnitrace_set_env(const char* env_name, const char* env_val)
 {