job control: reuse common job code for rpc jobs

This makes stderr and exit callbacks work for rpc jobs

src/nvim/globals.h

@@ -1244,6 +1244,9 @@ EXTERN char *ignoredp;
 // If a msgpack-rpc channel should be started over stdin/stdout
 EXTERN bool embedded_mode INIT(= false);
 
+/// next free id for a job or rpc channel
+EXTERN uint64_t next_chan_id INIT(= 1);
+
 /// Used to track the status of external functions.
 /// Currently only used for iconv().
 typedef enum {