Deprecate --enable-async-dns6

The IPv6 asynchronous name resolver is enabled if the host has at
least one interface with IPv6 address configured (the loopback address
will not be counted), which is roughly the same behaviour of the
standard getaddrinfo(3). To disable IPv6 asynchronous name resolver,
use --disable-ipv6.
Remove --enable-async-dns6 from manual
Remove deprecated options: --enable-direct-io and --metalink-servers