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_bc5.mak b/src/Make_bc5.mak
index 62d0397..a6b4bc9 100644
--- a/src/Make_bc5.mak
+++ b/src/Make_bc5.mak
@@ -565,6 +565,7 @@
 	$(OBJDIR)\term.obj \
 	$(OBJDIR)\ui.obj \
 	$(OBJDIR)\undo.obj \
+	$(OBJDIR)\usercmd.obj \
 	$(OBJDIR)\userfunc.obj \
 	$(OBJDIR)\version.obj \
 	$(OBJDIR)\window.obj \