* pinfo.h (EXITCODE_OK): Define new constant.

* sigproc.cc (child_info::sync): Return EXITCODE_OK if entering with exit_code
== 0.
(sig_send): Don't complain if sending signals while blocked if the sender isn't
in the main thread.

winsup/cygwin/pinfo.h

@@ -35,6 +35,7 @@ enum picom
 #define EXITCODE_SET	0x8000000
 #define EXITCODE_NOSET	0x4000000
 #define EXITCODE_RETRY	0x2000000
+#define EXITCODE_OK	0x1000000
 
 class fhandler_pipe;