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

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;