patch 8.0.0613: the conf filetype is used before ftdetect from packages

Problem:    The conf filetype detection is done before ftdetect scripts from
            packages that are added later.
Solution:   Add the FALLBACK argument to :setfiletype. (closes #1679,
            closes #1693)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 13d5fe4..2c9c878 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -12172,13 +12172,22 @@
 }
 
 /*
- * ":setfiletype {name}"
+ * ":setfiletype [FALLBACK] {name}"
  */
     static void
 ex_setfiletype(exarg_T *eap)
 {
     if (!did_filetype)
-	set_option_value((char_u *)"filetype", 0L, eap->arg, OPT_LOCAL);
+    {
+	char_u *arg = eap->arg;
+
+	if (STRNCMP(arg, "FALLBACK ", 9) == 0)
+	    arg += 9;
+
+	set_option_value((char_u *)"filetype", 0L, arg, OPT_LOCAL);
+	if (arg != eap->arg)
+	    did_filetype = FALSE;
+    }
 }
 #endif