MDEV-32188 make TIMESTAMP use whole 32-bit unsigned range

This patch extends the timestamp from
2038-01-19 03:14:07.999999 to 2106-02-07 06:28:15.999999
for 64 bit hardware and OS where 'long' is 64 bits.
This is true for 64 bit Linux but not for Windows.

This is done by treating the 32 bit stored int as unsigned instead of
signed.  This is safe as MariaDB has never accepted dates before the epoch
(1970).
The benefit of this approach that for normal timestamp the storage is
compatible with earlier version.

However for tables using system versioning we before stored a
timestamp with the year 2038 as the 'max timestamp', which is used to
detect current values.  This patch stores the new 2106 year max value
as the max timestamp. This means that old tables using system
versioning needs to be updated with mariadb-upgrade when moving them
to 11.4. That will be done in a separate commit.

include/mysql_time.h

@@ -19,15 +19,14 @@
 
 /*
   Portable time_t replacement.
-  Should be signed and hold seconds for 1902 -- 2038-01-19 range
-  i.e at least a 32bit variable
+  For 32 bit systems holds seconds for 1970 - 2038-01-19 range
+  For 64 bit systems (where long is 64 bit) holds seconds for 1970 - 2106
 
   Using the system built in time_t is not an option as
   we rely on the above requirements in the time functions   
 */
 typedef long my_time_t;
 
-
 /*
   Time declarations shared between the server and client API:
   you should not add anything to this header unless it's used