wall: access FILE under real user's credentials

While at it, move applet/config/kbuild bits into wall.c.
(This way, it's more visible that applet is suid'ed).

function                                             old     new   delta
wall_main                                             87     138     +51

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

miscutils/Config.src

@@ -591,13 +591,6 @@ config VOLNAME
 	help
 	  Prints a CD-ROM volume name.
 
-config WALL
-	bool "wall"
-	default y
-	depends on FEATURE_UTMP
-	help
-	  Write a message to all users that are logged in.
-
 config WATCHDOG
 	bool "watchdog"
 	default y