Fire/Frost Brand vs rusting

Implement the suggestion that Fire Brand avoids damage from rust traps
by boiling away the water.  Rather than making this be trap-specific, it
applies to all types of erosion which go through erode_obj().  That includes
hitting rust monsters but not dipping into potions or fountains, nor falling
into moats.  And it doesn't provide 100% protection, just a high chance of
avoiding rust damage.  Also, Frost Brand gets similar protection by freezing.

     The message handling needed some rewriting for the branch version.
That compiles ok but hasn't been tested.  It would have been simpler just to
move Yobjnam2() over even if nothing else was changed to use it yet....

doc/fixes34.4

@@ -113,7 +113,7 @@ armor which auto-curses when worn by hero should do same if worn by monster
 limit how high accuracy, damage, or protection can become via eating rings
 when blinded hero detects a trap by touch, make sure it shows up on the map
 confused remove curse will cause loss of knowledge of items' curse/bless state
-With astral vision, the ";" command should only display "normal vision"
+with astral vision, the ";" command should only display "normal vision"
 	for things that could be seen without astral vision
 
 
@@ -135,6 +135,7 @@ when adding an item to inventory, try to stack it with the quiver slot
 #adjust can be used to split an inventory stack
 cockatrice meat has a distinct flavor to some
 wish request for "<something> armor" will match item named "<something> mail"
+Fire Brand and Frost Brand have a chance to avoid taking rust damage
 
 
 Platform- and/or Interface-Specific New Features