patch 8.2.1898: command modifier parsing always uses global cmdmod

Problem:    Command modifier parsing always uses global cmdmod.
Solution:   Pass in cmdmod_T to use.  Rename struct fields consistently.
diff --git a/src/gui.c b/src/gui.c
index 5ae29fa..3f8f5fe 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -864,10 +864,10 @@
     // Only exit when there are no changed files
     exiting = TRUE;
 # ifdef FEAT_BROWSE
-    cmdmod.browse = TRUE;
+    cmdmod.cmod_flags |= CMOD_BROWSE;
 # endif
 # if defined(FEAT_GUI_DIALOG) || defined(FEAT_CON_DIALOG)
-    cmdmod.confirm = TRUE;
+    cmdmod.cmod_flags |= CMOD_CONFIRM;
 # endif
     // If there are changed buffers, present the user with a dialog if
     // possible, otherwise give an error message.