RFC: Activate IPv6 getaddrinfo, and return all addresses

[ihnorton]

ref #9892

before:

julia> getaddrinfo("localhost")
ip"127.0.0.1"  #could be either IPv4 or IPv6 based on host OS, and OS settings

after:

julia> getaddrinfo("localhost")
2-element Array{Base.IPAddr,1}:
 ip"::1"
 ip"127.0.0.1"

This is the opposite of my suggestion last night. But, after some reading, I believe this is the expected behavior of this function on both POSIX and Windows. This would be a breaking change for functions that rely on the current behavior of connect!, which is to take whatever address the system provides first. I'm punting on changing all of that until I get some opinions on whether it is worth fixing the API now to fully support IPv6, or continuing to put it off until someone actually needs it. @vtjnash had some concerns about OS inconsistencies last night on IRC.

Could just do the minimum to fix the broken test in #9892 instead.

@vtjnash @amitmurthy @Keno

[eschnett]

I believe the Unix getaddrinfo has an option to specify whether IPv4 and/or IPv6 addresses should be returned. Of course, such a flag can be added later

[vtjnash]

I think this is covered by #23596