remove time_t from struct you (trunk only)

There was an issue reported where save files between different
versions of a manufacturer's compiler were incompatible because the time_t
ubirthday field was changed from 32 bits to 64 bits.

32 bit time_t implementations will break at 19:14:07 on  January 18, 2038.
64 bit time_t implementations will break at 23:59:59 on December 31, 3000.

This removes the dependency on the size of time_t from the save file.
The ubirthday field is no longer embedded in struct you.
This also adds two general purpose routines to hacklib.c, one to convert a time
value to a 14 character char representation and the other to convert that
back to time_t. Those are used by the save/restore routines.

This is a savefile breaking change, so editlevel in patchlevel.h was
incremented.

src/shk.c

@@ -1847,7 +1847,7 @@ register struct monst *shkp;	/* if angry, impose a surcharge */
 		    /* get a value that's 'random' from game to game, but the
 		       same within the same game */
 		    boolean pseudorand =
-			(((int)u.ubirthday % obj->otyp) >= obj->otyp/2);
+			(((int)ubirthday % obj->otyp) >= obj->otyp/2);
 
 		    /* all gems are priced high - real or not */
 		    switch(obj->otyp - LAST_GEM) {