optimize pline("%s",string)

pline() already skips sprintf/vsnprintf if the format doesn't contain
any percent signs.  Do the same if the entire format is "%s".

Also, if ESC is used to suppress messages for the rest of the current
move, the text for any impossible warnings issued during that time
would be suppressed too so not be seen.

doc/fixes3-7-0.txt

@@ -1149,6 +1149,8 @@ prevent hug attacks and touch or engulf attacks for wrap, stick-to, and
 wand of speed gives temporary speed, potion gives intrinsic
 some monsters (riders, shopkeepers, priests, quest leader) can break boulders
 corpse-eating monsters will go out of their way to eat corpses on the floor
+warnings via impossible() would be unseen if message suppression via ESC at
+	--More-- prompt was in effect
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository