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/save.c

@@ -310,6 +310,7 @@ register int fd, mode;
 	if (u.ugold) (void)insert_gold_into_invent(FALSE);
 #endif
 	bwrite(fd, (genericptr_t) &u, sizeof(struct you));
+	bwrite(fd, yyyymmddhhmmss(ubirthday), 14);
 	save_killers(fd, mode);
 
 	/* must come before migrating_objs and migrating_mons are freed */