Replace magic constants for seek() calls in perl scripts

A couple of tests have been using 0 as magic constant while SEEK_SET can
be used instead.  This makes the code easier to understand, and more
consistent with the changes done in 3c5b0685.

Per discussion with Andrew Dunstan.

Discussion: https://postgr.es/m/YHrc24AgJQ6tQ1q0@paquier.xyz

contrib/amcheck/t/001_verify_heapam.pl

@@ -4,6 +4,7 @@ use warnings;
 use PostgresNode;
 use TestLib;
 
+use Fcntl qw(:seek);
 use Test::More tests => 80;
 
 my ($node, $result);
@@ -124,7 +125,7 @@ sub corrupt_first_page
 	# Corrupt some line pointers.  The values are chosen to hit the
 	# various line-pointer-corruption checks in verify_heapam.c
 	# on both little-endian and big-endian architectures.
-	seek($fh, 32, 0)
+	seek($fh, 32, SEEK_SET)
 	  or BAIL_OUT("seek failed: $!");
 	syswrite(
 		$fh,

src/bin/pg_amcheck/t/003_check.pl

@@ -3,6 +3,8 @@ use warnings;
 
 use PostgresNode;
 use TestLib;
+
+use Fcntl qw(:seek);
 use Test::More tests => 63;
 
 my ($node, $port, %corrupt_page, %remove_relation);
@@ -84,7 +86,7 @@ sub corrupt_first_page
 	# Corrupt some line pointers.  The values are chosen to hit the
 	# various line-pointer-corruption checks in verify_heapam.c
 	# on both little-endian and big-endian architectures.
-	seek($fh, 32, 0)
+	seek($fh, 32, SEEK_SET)
 		or BAIL_OUT("seek failed: $!");
 	syswrite(
 		$fh,

src/bin/pg_amcheck/t/004_verify_heapam.pl

@@ -4,6 +4,7 @@ use warnings;
 use PostgresNode;
 use TestLib;
 
+use Fcntl qw(:seek);
 use Test::More;
 
 # This regression test demonstrates that the pg_amcheck binary correctly
@@ -95,7 +96,7 @@ sub read_tuple
 {
 	my ($fh, $offset) = @_;
 	my ($buffer, %tup);
-	seek($fh, $offset, 0)
+	seek($fh, $offset, SEEK_SET)
 		or BAIL_OUT("seek failed: $!");
 	defined(sysread($fh, $buffer, HEAPTUPLE_PACK_LENGTH))
 		or BAIL_OUT("sysread failed: $!");
@@ -172,7 +173,7 @@ sub write_tuple
 					$tup->{c_va_extinfo},
 					$tup->{c_va_valueid},
 					$tup->{c_va_toastrelid});
-	seek($fh, $offset, 0)
+	seek($fh, $offset, SEEK_SET)
 		or BAIL_OUT("seek failed: $!");
 	defined(syswrite($fh, $buffer, HEAPTUPLE_PACK_LENGTH))
 		or BAIL_OUT("syswrite failed: $!");

src/bin/pg_basebackup/t/010_pg_basebackup.pl

@@ -4,6 +4,7 @@ use Cwd;
 use Config;
 use File::Basename qw(basename dirname);
 use File::Path qw(rmtree);
+use Fcntl qw(:seek);
 use PostgresNode;
 use TestLib;
 use Test::More tests => 110;
@@ -555,7 +556,7 @@ my $block_size      = $node->safe_psql('postgres', 'SHOW block_size;');
 # induce corruption
 system_or_bail 'pg_ctl', '-D', $pgdata, 'stop';
 open $file, '+<', "$pgdata/$file_corrupt1";
-seek($file, $pageheader_size, 0);
+seek($file, $pageheader_size, SEEK_SET);
 syswrite($file, "\0\0\0\0\0\0\0\0\0");
 close $file;
 system_or_bail 'pg_ctl', '-D', $pgdata, 'start';
@@ -574,7 +575,7 @@ open $file, '+<', "$pgdata/$file_corrupt1";
 for my $i (1 .. 5)
 {
 	my $offset = $pageheader_size + $i * $block_size;
-	seek($file, $offset, 0);
+	seek($file, $offset, SEEK_SET);
 	syswrite($file, "\0\0\0\0\0\0\0\0\0");
 }
 close $file;
@@ -591,7 +592,7 @@ rmtree("$tempdir/backup_corrupt2");
 # induce corruption in a second file
 system_or_bail 'pg_ctl', '-D', $pgdata, 'stop';
 open $file, '+<', "$pgdata/$file_corrupt2";
-seek($file, $pageheader_size, 0);
+seek($file, $pageheader_size, SEEK_SET);
 syswrite($file, "\0\0\0\0\0\0\0\0\0");
 close $file;
 system_or_bail 'pg_ctl', '-D', $pgdata, 'start';

src/bin/pg_checksums/t/002_actions.pl

@@ -5,6 +5,8 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
+
+use Fcntl qw(:seek);
 use Test::More tests => 63;
 
 
@@ -50,7 +52,7 @@ sub check_relation_corruption
 
 	# Time to create some corruption
 	open my $file, '+<', "$pgdata/$file_corrupted";
-	seek($file, $pageheader_size, 0);
+	seek($file, $pageheader_size, SEEK_SET);
 	syswrite($file, "\0\0\0\0\0\0\0\0\0");
 	close $file;