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>

scripts/kconfig/streamline_config.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl
 #
 # Copyright 2005-2009 - Steven Rostedt
 # Licensed under the terms of the GNU GPL License version 2
@@ -42,6 +42,7 @@
 #    mv config_strip .config
 #    make oldconfig
 #
+use warnings;
 use strict;
 use Getopt::Long;