MEDIUM: sample: Add IPv6 support to the ipmask converter

MEDIUM: sample: Add IPv6 support to the ipmask converter

Add an optional second parameter to the ipmask converter that specifies
the number of bits to mask off IPv6 addresses.

If the second parameter is not given IPv6 addresses fail to mask (resulting
in an empty string), preserving backwards compatibility: Previously
a sample like `src,ipmask(24)` failed to give a result for IPv6 addresses.

This feature can be tested like this:

  defaults
  	log	global
  	mode	http
  	option	httplog
  	option	dontlognull
  	timeout connect 5000
  	timeout client  50000
  	timeout server  50000

  frontend fe
  	bind :::8080 v4v6

  	# Masked IPv4 for IPv4, empty for IPv6 (with and without this commit)
  	http-response set-header Test %[src,ipmask(24)]
  	# Correctly masked IP addresses for both IPv4 and IPv6
  	http-response set-header Test2 %[src,ipmask(24,ffff:ffff:ffff:ffff::)]
  	# Correctly masked IP addresses for both IPv4 and IPv6
  	http-response set-header Test3 %[src,ipmask(24,64)]

  	default_backend be

  backend be
  	server s example.com:80

Tested-By: Jarno Huuskonen <jarno.huuskonen@uef.fi>