Use perl warnings pragma consistently

We've had a mixture of the warnings pragma, the -w switch on the shebang
line, and no warnings at all. This patch removes the -w swicth and add
the warnings pragma to all perl sources missing it. It raises the
severity of the TestingAndDebugging::RequireUseWarnings  perlcritic
policy to level 5, so that we catch any future violations.

Discussion: https://postgr.es/m/20200412074245.GB623763@rfd.leadboat.com

Author: adunstan

contrib/intarray/bench/bench.pl

@@ -1,6 +1,7 @@
 #!/usr/bin/perl
 
 use strict;
+use warnings;
 
 # make sure we are in a sane environment.
 use DBI();

contrib/intarray/bench/create_test.pl

@@ -3,6 +3,8 @@
 # contrib/intarray/bench/create_test.pl
 
 use strict;
+use warnings;
+
 print <<EOT;
 create table message (
 	mid	int not null,

contrib/seg/seg-validate.pl

@@ -1,6 +1,7 @@
 #!/usr/bin/perl
 
 use strict;
+use warnings;
 
 my $integer = '[+-]?[0-9]+';
 my $real    = '[+-]?[0-9]+\.[0-9]+';

contrib/seg/sort-segments.pl

@@ -3,6 +3,7 @@
 # this script will sort any table with the segment data type in its last column
 
 use strict;
+use warnings;
 
 my @rows;
 

doc/src/sgml/mk_feature_tables.pl

@@ -1,8 +1,9 @@
-# /usr/bin/perl -w
+# /usr/bin/perl
 
 # doc/src/sgml/mk_feature_tables.pl
 
 use strict;
+use warnings;
 
 my $yesno = $ARGV[0];
 

src/backend/catalog/genbki.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 #----------------------------------------------------------------------
 #
 # genbki.pl

src/backend/utils/Gen_fmgrtab.pl

@@ -1,4 +1,4 @@
-#! /usr/bin/perl -w
+#! /usr/bin/perl
 #-------------------------------------------------------------------------
 #
 # Gen_fmgrtab.pl

src/backend/utils/mb/Unicode/UCS_to_BIG5.pl

@@ -25,6 +25,8 @@
 #		 # and Unicode name (not used in this script)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_BIG5.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_CN.pl

@@ -14,6 +14,8 @@
 # and the "b" field is the hex byte sequence for GB18030
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_CN.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_JIS_2004.pl

@@ -8,6 +8,8 @@
 # "euc-jis-2004-std.txt" (http://x0213.org)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_JIS_2004.pl';