fix github issue #531 - genderless corpses

Dead monsters that had traits saved with the corpse would revive as
the same gender, but ordinary corpses revived with random gender so
could be different from before they got killed.

Since corpses of monsters lacked gender, those for monsters with
gender-specific names were described by the neuter name.

This is a fairly big change for a fairly minor problem and needs a
lot more testing.

Fixes #531

include/obj.h

@@ -55,10 +55,8 @@ struct obj {
                 * candy bar wrapper index;
                 * scroll of mail (normal==0, bones or wishing==1, written==2);
                 * splash of venom (normal==0, wishing==1);
-                * historic flag and gender for statues */
-#define STATUE_HISTORIC 0x01
-#define STATUE_MALE 0x02
-#define STATUE_FEMALE 0x04
+                * gender for corpses and statues (0..3, CORPSTAT_GENDER),
+                * historic flag (4, CORPSTAT_HISTORIC) for statues */
     char oclass;    /* object class */
     char invlet;    /* designation in inventory */
     char oartifact; /* artifact array index */