internal/outcome/shim: move signal constants

The magic numbers hurt readability.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/outcome/shim-signal.h

@@ -1,3 +1,11 @@
 #include <signal.h>
 
+/* see shim.go for documentation */
+typedef enum {
+  HAKUREI_SHIM_EXIT_REQUESTED,
+  HAKUREI_SHIM_ORPHAN,
+  HAKUREI_SHIM_INVALID,
+  HAKUREI_SHIM_BAD_PID,
+} hakurei_shim_msg;
+
 void hakurei_shim_setup_cont_signal(pid_t ppid, int fd);