IPv6 support

[mildred]

When the server is IPv6 only, connect fails:

Error: unhandled exception: Additional info: Name or service not known
Async traceback:
  /home/mildred/Projets/sqlemail.nim/src/submit_email.nim(112) submit_email
  /home/mildred/Projets/sqlemail.nim/src/submit_email.nim(54) send (Async)
  /home/mildred/.nimble/pkgs2/smtp-0.1.0-df9380a905e6f2f9f4228474077ca15f2ccbde29/smtp.nim(254) connect
  /home/mildred/.choosenim/toolchains/nim-2.0.0/lib/pure/net.nim(2055) connect
  /home/mildred/.choosenim/toolchains/nim-2.0.0/lib/pure/nativesockets.nim(311) getAddrInfo
  /home/mildred/.choosenim/toolchains/nim-2.0.0/lib/std/oserrors.nim(92) raiseOSError
Exception message: Additional info: Name or service not known

edit: sorry, it might be my server down instead

[mildred]

Problem seems to be in asyncdispatch where getaddrinfo is called with AF_INET instead of AF_UNSPEC

[mildred]

Reopening issue. See nim-lang/Nim#22442 and forum thread. The problem is that the smtp package uses newSocket and then connect whereas to have an IPv6 compatible library one should use dial which performs the address lookup and then created the socket using the correct address family depending on if the address is IPv4 or IPv6.