Add function regexp_quote().

This was originally commit a154774bba6e in nsc-puppet-utils.

Lib/puppet/parser/functions/regexp_quote.rb

+module Puppet::Parser::Functions
+    newfunction(:regexp_quote, :type => :rvalue) do |args|
+	if args.length != 1
+	    self.fail("regexp_quote(): wrong number of arguments" +
+		      " (#{args.length} for 1)")
+	end
+	return Regexp.quote(args[0])
+    end
+end