patch 9.0.1231: completion of :runtime does not handle {where} argument

Problem:    Completion of :runtime does not handle {where} argument.
Solution:   Parse the {where} argument. (closes #11863)
diff --git a/src/vim.h b/src/vim.h
index 930b17b..ec08f63 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2662,7 +2662,7 @@
 #define DIP_NORTP   0x20	// do not use 'runtimepath'
 #define DIP_NOAFTER 0x40	// skip "after" directories
 #define DIP_AFTER   0x80	// only use "after" directories
-#define DIP_PRNEXT  0x100	// for print also file extension
+#define DIP_KEEPEXT  0x100	// for completion: include file extension
 
 // Lowest number used for window ID. Cannot have this many windows.
 #define LOWEST_WIN_ID 1000