This modernizes acquire_terminal() in a couple of ways:

1. The three boolean arguments are replaced by a flags parameter, that
   should be more descriptive in what it does.

2. We now properly handle inotify queue overruns

3. We use _cleanup_ for closing the fds now, to shorten the code quite a
   bit.

Behaviour should not be altered by this.

…hed terminal features

Let's forget all relevant terminal features we learnt when we make a
console or /dev/null stdin/stdout/stderr.

Also, while we are at it, let's drop the various _unlikely_ and
_likely_ annotiations around the terminal feature caches. In many cases
we call the relevant functions only once in which cases the annotations
are likely to do just harm and no good. After all we can't know if the
specific code will call us just once or many times...

… not an int

We should be careful with these types, and if we do convert between
"int" and "ssize_t" we should do so explicitly rather than implicitly.
Otherwise this just looks like a bug.

We should rather sleep to much than too little. This otherwise might
result in a busy loop, because we slept too little and then recheck
again coming to the conclusion we need to go to sleep again, and so on.

It's prettier that way!

We already have the terminal open, hence pass the fd we got to
ask_password_tty(), so that it doesn't have to reopen it a second time.

This is mostly an optimization, but it has the nice benefit of making us
independent from RLIMIT_NOFILE issues and so on, as we don't need to
allocate another fd needlessly.

The password prompt used to be highlighted, and that was a good thing.
Let's fix things to make the prompt highlighted again.

Fixes: systemd#3853

Also, make sure when we run in a container, we don't use the data from
/sys at all, but immediately fall back to /dev/console itself.

Let's normalize the behaviour: return a negative errno style error code,
and return the resolved string directly as argument.

…n a string

A couple of fixes:

1. always bzero_explicit() away what we remove from the passphrase
   buffer. The UTF-8 code assumes the string remains NUL-terminated, and
   we hence should enforce that. memzero() would do too here, but let's
   be paranoid after all this is key material.

2. when clearing '*' characters from string, do so counting UTF-8
   codepoints properly. We already have code in place to count UTF-8
   codepoints when generating '*' characters, hence we should take the
   same care when clearing them again.

3. Treat NUL on input as an alternative terminator to newline or EOF.

4. When removing characters from the password always also reset the
   "codepoint" index properly.