app/acl: support different formats for IPv6 address

app/acl: support different formats for IPv6 address

Within ACL rule IPv6 address can be represented in different ways:
either as 4x4B fields, or as 2x8B fields.
Till now, only first format was supported.
Extend test-acl to support both formats, mainly for testing and
demonstrating purposes.
To control desired behavior '--ipv6' command-line option is extended
to accept an optional argument:
To be more precise:
'--ipv6'    - use 4x4B fields format (default behavior)
'--ipv6=4B' - use 4x4B fields format (default behavior)
'--ipv6=8B' - use 2x8B fields format

Also replaced home brewed IPv4/IPv6 address parsing with inet_pton() calls.

Signed-off-by: Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>