Add shields of shock resistance and of drain resistance

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

src/do_wear.c

@@ -706,10 +706,12 @@ Shield_on(void)
 {
     /* no shield currently requires special handling when put on, but we
        keep this uncommented in case somebody adds a new one which does
-       [reflection is handled by setting u.uprops[REFLECTION].extrinsic
+       [the magical shields are handled by setting u.uprops[*].extrinsic
        in setworn() called by armor_or_accessory_on() before Shield_on()] */
     switch (uarms->otyp) {
     case SMALL_SHIELD:
+    case SHIELD_OF_DRAIN_RESISTANCE:
+    case SHIELD_OF_SHOCK_RESISTANCE:
     case ELVEN_SHIELD:
     case URUK_HAI_SHIELD:
     case ORCISH_SHIELD:
@@ -736,6 +738,8 @@ Shield_off(void)
        keep this uncommented in case somebody adds a new one which does */
     switch (uarms->otyp) {
     case SMALL_SHIELD:
+    case SHIELD_OF_DRAIN_RESISTANCE:
+    case SHIELD_OF_SHOCK_RESISTANCE:
     case ELVEN_SHIELD:
     case URUK_HAI_SHIELD:
     case ORCISH_SHIELD:

src/u_init.c

@@ -603,8 +603,10 @@ knows_class(char sym)
      */
 
     for (ct = svb.bases[(uchar) sym]; ct < svb.bases[(uchar) sym + 1]; ct++) {
-        /* not flagged as magic but shouldn't be pre-discovered */
-        if (ct == CORNUTHAUM || ct == DUNCE_CAP)
+        /* not flagged as magic but shouldn't be pre-discovered
+           (small shields look the same as two types of magical shield;
+           cornuthaum / dunce cap look the same as each other) */
+        if (ct == CORNUTHAUM || ct == DUNCE_CAP || ct == SMALL_SHIELD)
             continue;
         if (sym == WEAPON_CLASS) {
             odummy.otyp = ct; /* update 'o' */