patch 8.0.0899: function name mch_stop_job() is confusing

Problem:    Function name mch_stop_job() is confusing.
Solution:   Rename to mch_signal_job().
diff --git a/src/os_unix.c b/src/os_unix.c
index ade8d88..141a3f0 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -5557,7 +5557,7 @@
  * Return FAIL if "how" is not a valid name.
  */
     int
-mch_stop_job(job_T *job, char_u *how)
+mch_signal_job(job_T *job, char_u *how)
 {
     int	    sig = -1;
     pid_t   job_pid;