Add exit codes to qrexec.h

They were open-coded all over the place, and status 126 ("Request
refused") used when other statuses would be more appropriate.

This just adds the exit codes.  It doesn't change any existing behavior.

libqrexec/qrexec.h

@@ -173,4 +173,11 @@ enum {
 // support only very small configuration files,
 #define MAX_CONFIG_SIZE 4096
 
+// Exit codes
+
+// Service call refused
+#define QREXEC_EXIT_REQUEST_REFUSED 126
+// Problem with qrexec itself
+#define QREXEC_EXIT_PROBLEM 125
+
 #endif /* QREXEC_H */