patch 8.1.0032: BS in prompt buffer starts new line

Problem:    BS in prompt buffer starts new line.
Solution:   Do not allows BS over the prompt.  Make term_sendkeys() handle
            special keys. Add a test.

src/option.c

@@ -12439,6 +12439,10 @@ check_opt_wim(void)
 can_bs(
     int		what)	    /* BS_INDENT, BS_EOL or BS_START */
 {
+#ifdef FEAT_JOB_CHANNEL
+    if (what == BS_START && bt_prompt(curbuf))
+	return FALSE;
+#endif
     switch (*p_bs)
     {
 	case '2':	return TRUE;