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_sas.mak b/src/Make_sas.mak
index 78d6d2a..0d8eb3d 100644
--- a/src/Make_sas.mak
+++ b/src/Make_sas.mak
@@ -146,6 +146,7 @@
 	term.c \
 	ui.c \
 	undo.c \
+	usercmd.c \
 	userfunc.c \
 	window.c \
 	version.c
@@ -208,6 +209,7 @@
 	term.o \
 	ui.o \
 	undo.o \
+	usercmd.o \
 	userfunc.o \
 	window.o \
 	$(TERMLIB)
@@ -271,6 +273,7 @@
 	proto/termlib.pro \
 	proto/ui.pro \
 	proto/undo.pro \
+	proto/usercmd.pro \
 	proto/userfunc.pro \
 	proto/window.pro
 
@@ -445,6 +448,8 @@
 proto/ui.pro:		ui.c
 undo.o:			undo.c
 proto/undo.pro:		undo.c
+usercmd.o:		usercmd.c
+proto/usercmd.pro:	usercmd.c
 userfunc.o:		userfunc.c
 proto/userfunc.pro:	userfunc.c
 window.o:		window.c