Allow pg_archivecleanup to remove backup history files

Hi,

Currently pg_archivecleanup doesn't remove backup history files even
when they're older than oldestkeptwalfile.

Of course the size of backup history files are smaller than WAL files
and they wouldn't consume much disk space, but a lot of backup history
files(e.g. daily backup for years) whose backups data have been already
removed are unnecessary and I would appreciate if pg_archivecleanup has
an option to remove them.

Attached a PoC patch, which added new option -b to remove files
including backup history files older than oldestkeptwalfile.

$ ls archivedir
000000010000000000000001 000000010000000000000003
000000010000000000000006
000000010000000000000008
000000010000000000000002 000000010000000000000004
000000010000000000000007
000000010000000000000009
000000010000000000000002.00000028.backup 000000010000000000000005
000000010000000000000007.00000028.backup
00000001000000000000000A.partial

$ pg_archivecleanup -b archivedir 000000010000000000000009

$ ls archivedir
000000010000000000000009 00000001000000000000000A.partial

Any thoughts?

--
Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION