Simplify process_wait()

The function process_wait() returned a bool (true if the process
terminated successfully) and provided the exit code via an output
parameter exit_code.

But the returned value was always equivalent to exit_code == 0, so just
return the exit code instead.

app/src/util/process.h

@@ -47,12 +47,12 @@ bool
 process_terminate(process_t pid);
 
 // wait and close the process (like waitpid())
-bool
-process_wait(process_t pid, exit_code_t *exit_code);
+exit_code_t
+process_wait(process_t pid);
 
 // wait (but does not close) the process (waitid() with WNOWAIT)
-bool
-process_wait_noclose(process_t pid, exit_code_t *exit_code);
+exit_code_t
+process_wait_noclose(process_t pid);
 
 // close the process
 //