Get rid of a spurious warning on the console when booting the kernel

from the interactive loader(8) prompt and beastie_disable="YES" is set
in loader.conf(5). In this case menu.rc is not evaluated and consequently
menu-unset does not have a body yet. This results in the ficl warning
"menu-unset not found" when try-menu-unset invokes menu-unset.

Check for beastie_disable="YES" explicitly, so that the try-menu-unset
word will not attempt to invoke menu-unset because the menu will have
never been configured. [1]
Use the sfind primitive as a last resort as an additional safer approach
conjuring a foreign word safely. [2]

PR:		kern/163938
Submitted by:	Devin Teske [1]
Reviewed by:	Devin Teske [2]
Reported and tested by:	dim
MFC after:	1 week
X-MFC with:	r228985

sys/boot/forth/loader.4th

@@ -44,9 +44,20 @@ include /boot/support.4th
 only forth also support-functions also builtins definitions
 
 : try-menu-unset
+  \ menu-unset may not be present
+  s" beastie_disable" getenv
+  dup -1 <> if
+    s" YES" compare-insensitive 0= if
+      exit
+    then
+  else
+    drop
+  then
   s" menu-unset"
-  ['] evaluate catch if
-    2drop
+  sfind if
+    execute
+  else
+    drop
   then
 ;