BUG #16780: Inconsistent recovery_target_xid handling across platforms

The following bug has been logged on the website:

Bug reference: 16780
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 13.1
Operating system: Windows
Description:

While testing src/test/recovery with a non-zero epoch the inconsistency was
discovered.
The 003_recovery_targets.pl test with the slight modification passes on
Linux x86_64, but fails on Windows x64.
The modification is to append just after the $node_master->init:
command([ 'pg_resetwal', '--epoch=1', $node_master->data_dir ]);
And on Windows I get:
t/003_recovery_targets.pl ............ 2/9 Bailout called. Further testing
stopped: pg_ctl start failed
FAILED--Further testing stopped: pg_ctl start failed

regress_log_003_recovery_targets contains:
# pg_ctl start failed; logfile:
2020-12-18 12:25:11.597 MSK [1312] LOG: invalid value for parameter
"recovery_target_xid": "4294967782"
2020-12-18 12:25:11.597 MSK [1312] FATAL: configuration file
".../src/test/recovery/tmp_check/t_003_recovery_targets_standby_2_data/pgdata/postgresql.conf"
contains errors

So on Windows the recovery_target_xid parameter doesn't accept 64-bit
value.
As I see in check_recovery_target_xid() the strtoul() function is used to
check/convert the input value, that is later stored into 32-bit variable.
Thus, on platforms where unsigned long is 32-bit wide, you can't specify a
result of pg_current_xact_id() as recovery_target_xid, while on platforms
where unsigned long is 64-bit, you can, but high 32 bits are just ignored.