change gy.youmonst

Add a pointer to struct you, and put umonst there. Eliminate
the youmonst struct in gy.

The naming convention better matches the other u related fields,
and u.umonst has the same level of indirection as other monst pointers.

The u.umonst pointer is cleared prior to saving the parent u (struct you),
and u.umonst is reestablished upon restore.

Invalidates existing saves and bones due to the presence of the added
pointer field in struct you.

include/you.h

@@ -496,8 +496,9 @@ struct you {
     struct skills weapon_skills[P_NUM_SKILLS];
     boolean twoweap;         /* KMH -- Using two-weapon combat */
     short mcham;             /* vampire mndx if shapeshifted to bat/cloud */
-    short umovement;         /* instead of youmonst.movement */
+    short umovement;         /* instead of u.umonst->movement */
     schar uachieved[N_ACH];  /* list of achievements in the order attained */
+    struct monst *umonst;
 }; /* end of `struct you' */