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/runtime/doc/various.txt b/runtime/doc/various.txt
index 428a734..923ac24 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -456,7 +456,8 @@
 N  *+timers*		the |timer_start()| function
 N  *+title*		Setting the window 'title' and 'icon'
 N  *+toolbar*		|gui-toolbar|
-N  *+user_commands*	User-defined commands. |user-commands|
+T  *+user_commands*	User-defined commands. |user-commands|
+			Always enabled since 8.1.1210.
 B  *+vartabs*		Variable-width tabstops. |'vartabstop'|
 N  *+viminfo*		|'viminfo'|
    *+vertsplit*		Vertically split windows |:vsplit|; Always enabled