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_manx.mak b/src/Make_manx.mak
index 6eb7bfa..e53522f 100644
--- a/src/Make_manx.mak
+++ b/src/Make_manx.mak
@@ -93,6 +93,7 @@
 	term.c \
 	ui.c \
 	undo.c \
+	usercmd.c \
 	userfunc.c \
 	window.c \
 	version.c
@@ -156,6 +157,7 @@
 	obj/term.o \
 	obj/ui.o \
 	obj/undo.o \
+	obj/usercmd.o \
 	obj/userfunc.o \
 	obj/window.o \
 	$(TERMLIB)
@@ -218,6 +220,7 @@
 	proto/termlib.pro \
 	proto/ui.pro \
 	proto/undo.pro \
+	proto/usercmd.pro \
 	proto/userfunc.pro \
 	proto/window.pro
 
@@ -443,6 +446,9 @@
 obj/undo.o:	undo.c
 	$(CCSYM) $@ undo.c
 
+obj/usercmd.o:	usercmd.c
+	$(CCSYM) $@ usercmd.c
+
 obj/userfunc.o:	userfunc.c
 	$(CCSYM) $@ userfunc.c