patch 8.1.1210: support for user commands is spread out

Problem:    Support for user commands is spread out. No good reason to make
            user commands optional.
Solution:   Move user command support to usercmd.c.  Always enable the
            user_commands feature.
diff --git a/src/Make_ivc.mak b/src/Make_ivc.mak
index 08bd874..a8b9dff 100644
--- a/src/Make_ivc.mak
+++ b/src/Make_ivc.mak
@@ -269,6 +269,7 @@
 	"$(INTDIR)/term.obj" \
 	"$(INTDIR)/ui.obj" \
 	"$(INTDIR)/undo.obj" \
+	"$(INTDIR)/usercmd.obj" \
 	"$(INTDIR)/userfunc.obj" \
 	"$(INTDIR)/version.obj" \
 	"$(INTDIR)/window.obj"
@@ -728,6 +729,10 @@
 # End Source File
 # Begin Source File
 
+SOURCE=.\usercmd.c
+# End Source File
+# Begin Source File
+
 SOURCE=.\userfunc.c
 # End Source File
 # Begin Source File