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/decl.c

@@ -137,6 +137,7 @@ NEARDATA struct sysflag sysflags = DUMMY;
 #endif
 NEARDATA struct instance_flags iflags = DUMMY;
 NEARDATA struct you u = DUMMY;
+NEARDATA time_t ubirthday = DUMMY;
 
 NEARDATA struct obj *invent = (struct obj *)0,
 	*uwep = (struct obj *)0, *uarm = (struct obj *)0,