scripts: Switch to more portable Perl shebang

The default NetBSD package manager is pkgsrc and it installs Perl
along other third party programs under custom and configurable prefix.
The default prefix for binary prebuilt packages is /usr/pkg, and the
Perl executable lands in /usr/pkg/bin/perl.

This change switches "/usr/bin/perl" to "/usr/bin/env perl" as it's
the most portable solution that should work for almost everybody.
Perl's executable is detected automatically.

This change switches -w option passed to the executable with more
modern "use warnings;" approach. There is no functional change to the
default behavior.

While there, drop "require 5" from scripts/namespace.pl (Perl from 1994?).

Signed-off-by: Kamil Rytarowski <n54@gmx.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/scripts/cleanpatch b/scripts/cleanpatch
index 9680d03..3e5a230 100755
--- a/scripts/cleanpatch
+++ b/scripts/cleanpatch
@@ -1,9 +1,10 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl
 #
 # Clean a patch file -- or directory of patch files -- of stealth whitespace.
 # WARNING: this can be a highly destructive operation.  Use with caution.
 #
 
+use warnings;
 use bytes;
 use File::Basename;