Add shields of shock resistance and of drain resistance

These both appear as "wooden shield" when unidentified; so does the
small shield.

doc/fixes3-7-0.txt

@@ -2904,6 +2904,7 @@ ring of stealth prevents hero from leaving tracks
 the game now automatically tracks which sell prices and buy prices you have
         seen for each type of item; these are visible in the discoveries list,
         and can also be shown elsewhere using the new 'price_quotes' option
+new shields: shield of shock resistance, shield of drain resistance
 
 
 Platform- and/or Interface-Specific New Features