Fix hang stracing forking processes but not following child

* ntdll.h (PROCESSINFOCLASS): Define ProcessDebugFlags.
        * sigproc.cc (child_info::child_info): Only propagate _CI_STRACED to
        child if strace is actually tracing child processes.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/ntdll.h

@@ -532,7 +532,8 @@ typedef enum _PROCESSINFOCLASS
   ProcessTimes = 4,
   ProcessSessionInformation = 24,
   ProcessWow64Information = 26,
-  ProcessImageFileName = 27
+  ProcessImageFileName = 27,
+  ProcessDebugFlags = 31
 } PROCESSINFOCLASS;
 
 /* Checked on 64 bit. */