Remove ipv4 only compile flag

[mjrider]

Current the code is default compiled in ipv4 only mode, and ipv6 is a feature that needs to be enabled on compile time

Assumption: Assume every modern os supports ipv6 and ipv4 only mode is no longer needed

Proposal:

• Remove all the ifdef checks on INET6, and delete the case when the code is compiled ipv4 only mode
• adapt configure.in that it fails if inet6 is not available or just remove the check completely everywhere

this remove 2 issues:

• opers make the mistake to create an oper without ipv6 support
• a chunk of code which is useless, makes the code easier to maintain or tested when new code is added that it still is running in ipv6 less systems

[patrick-irc]

I have continued your work.

1. There is an iauth option which was IPv4-only. I have added IPv6 support now.

    ip = ::1

2. I have removed lhex iauth module completly because it was disabled if INET6 was defined and I never heard that anyone uses it.
   See also:

• https://github.com/ircnet/ircd/blob/release/2.11.3/iauth/mod_lhex.c#L54
• https://ftp.be.debian.org/ftp.irc.org/server/LHEx/README

3. I have changed the configure script to always compile with IPv7 enabled (tested on Linux and FreeBSD).

Please do a code review and test it too.