Expand TLS-Crypt-V2-Key metadata to include IPv6 client IP filtering

In IPv6, Easy-TLS uses simple string matching, not methematical techniques.
This is due to the 128bit binary involved. (IPv4 is mathematically evaluated)

To match an IPv6 address to a client address, it is recommanded to use a
subnet/mask (mask <= 124), not a host/128.

Examples:

* Correct: 2000:1:2:3::/64, 2000:1:2:3:abcd::/80, 2000::1:2:3:4/128(+)
(+: Not recommended)
* Incorrect: 2000:1:2:3:abcd::/64, 2000::1:2:3:4/80, :2000:1:2:3:4::/64

Easy-TLS also has some new functions for checking valid IPv4/6:

* v4ip: Validate IPv4 address
* v6ip: Validate IPv6 address
* x6ip: Expand a compressed IPv6 address

Examples:

* ./easytls v4ip 1.2.3.4/24
* ./easytls v6ip 2000::c0ff:ee/64
* ./easytls x6ip 2000::c0ff:ee/64

Note: When expanding an IPv6 address, easytls also verifies that this is
a valid subnet, for use in TLS-Crypt-V2 metadata, according to the rules
outlined here. Thus, some valid IPv6 addresses are not valid in Easy-TLS.

Signed-off-by: Richard T Bonhomme <[email protected]>