Support configurable WAL segment size.

PostgreSQL 11 introduces configurable WAL segment sizes, from 1MB to 1GB.

There are two areas that needed to be updated to support this: building the archive-get queue and checking that WAL has been archived after a backup.  Both operations require the WAL segment size to properly build a list.

Checking the archive after a backup is still implemented in Perl and has an active database connection, so just get the WAL segment size from the database.

The archive-get command does not have a connection to the database, so get the WAL segment size from pg_control instead.  This requires a deeper inspection of pg_control than has been done in the past, so it seemed best to copy the relevant data structures from each version of PostgreSQL and build a generic interface layer to address them.  While this approach is a bit verbose, it has the advantage of being relatively simple, and can easily be updated for new versions of PostgreSQL.

Since the integration tests generate pg_control files for testing, teach Perl how to generate files with the correct offsets for both 32-bit and 64-bit architectures.

lib/pgBackRest/Backup/Backup.pm

@@ -723,6 +723,7 @@ sub process
     # Start backup (unless --no-online is set)
     my $strArchiveStart = undef;
     my $strLsnStart = undef;
+    my $iWalSegmentSize = undef;
     my $hTablespaceMap = undef;
 	my $hDatabaseMap = undef;
 
@@ -757,7 +758,7 @@ sub process
     else
     {
         # Start the backup
-        ($strArchiveStart, $strLsnStart) =
+        ($strArchiveStart, $strLsnStart, $iWalSegmentSize) =
             $oDbMaster->backupStart(
                 BACKREST_NAME . ' backup started at ' . timestampFormat(undef, $lTimestampStart), cfgOption(CFGOPT_START_FAST));
 
@@ -929,7 +930,7 @@ sub process
 
         my $oArchiveInfo = new pgBackRest::Archive::Info(storageRepo()->pathGet(STORAGE_REPO_ARCHIVE), true);
         my $strArchiveId = $oArchiveInfo->archiveId();
-        my @stryArchive = lsnFileRange($strLsnStart, $strLsnStop, $strDbVersion);
+        my @stryArchive = lsnFileRange($strLsnStart, $strLsnStop, $strDbVersion, $iWalSegmentSize);
 
         foreach my $strArchive (@stryArchive)
         {