patch 8.1.0281: parsing command modifiers is not separated

Problem:    Parsing command modifiers is not separated.
Solution:   Move command modifier parsing to a separate function.
diff --git a/src/globals.h b/src/globals.h
index 5876329..09820cf 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -678,8 +678,7 @@
 				 * changed, no buffer can be deleted and
 				 * current directory can't be changed.
 				 * Used for SwapExists et al. */
-#ifdef FEAT_EVAL
-# define HAVE_SANDBOX
+#ifdef HAVE_SANDBOX
 EXTERN int	sandbox INIT(= 0);
 				/* Non-zero when evaluating an expression in a
 				 * "sandbox".  Several things are not allowed