patch 9.0.0537: the do_set() function is much too long

Problem:    The do_set() function is much too long.
Solution:   Move setting of a string option to a separate function.
diff --git a/src/version.c b/src/version.c
index 75611d1..e6045d7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    537,
+/**/
     536,
 /**/
     535,