Unfortunately this can't be implemented "above the vtable" because
various operating systems don't provide low level DNS resolution
primitives such as just putting the list of nameservers in a file.
Without libc on Linux it works great though!
Anyway this also changes the API to be based on Io.Queue. By using a
large enough buffer, reusable code can be written that does not require
concurrent, yet takes advantage of responding to DNS queries as they
come in. I sketched out a new implementation of `HostName.connect` to
demonstrate this, but it will require an additional API (`Io.Select`) to
be implemented in a future commit.
This commit also introduces "uncancelable" variants for mutex locking,
waiting on a condition, and putting items into a queue.
In the future, it might be nice to introduce a type for file system path
names. This would be a way to avoid having InvalidFileName in the error
set, since construction of such type could validate it above the
interface.
Calling recvmsg first means no poll syscall needed when messages are
already in the operating system queue. Empirically, this happens when
repeating a DNS query that has been already been made recently. In such
case, poll() is never called!
extract pure functional logic into pure functions and then layer the
scope crap on top properly
the formatting code incorrectly didn't do the reverse operation
(if_indextoname). fix that with some TODO panics
