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.

include/you.h

@@ -355,8 +355,6 @@ struct you {
 #endif
 	int	umortality;		/* how many times you died */
 	int ugrave_arise; /* you die and become something aside from a ghost */
-	time_t	ubirthday;		/* real world time when game began */
-
 	int	weapon_slots;		/* unused skill slots */
 	int	skills_advanced;		/* # of advances made so far */
 	xchar	skill_record[P_SKILL_LIMIT];	/* skill advancements */