kbdint result vfunc may return various values, so use an enum to make it
clearer what each result means without having to dig into the struct
documentation.

Makes things more readable and easier to extend

SSH keyboard-interactive authentication method supports instructions but
sshd didn't show them until an user prompt was requested.

This is quite inconvenient for various PAM modules that need to notify
an user without requiring for their explicit input.

So, properly implement RFC4256 making instructions to be shown to users
when they are requested from PAM.

Closes: https://bugzilla.mindrot.org/show_bug.cgi?id=2876

…utf-8

As per the previous server change now the keyboard-interactive service
and instruction values could be reported as soon as they are available
and so they're not prompts anymore and not parsed like them.

While this was already supported by the SSH client, these messages were
not properly written as the escaped sequences they contained were not
correctly reported.

So for example a message containing "\" was represented as "\\" and
similarly for all the other C escape sequences.

This was leading to more problems when it come to utf-8 chars, as they
were only represented by their octal representation.

This was easily testable by adding a line like the one below to the
sshd PAM service:
  auth    requisite pam_echo.so Hello SSHD! Want some 🍕?

Which was causing this to be written instead:
  Hello SSHD! Want some \360\237\215\225?

To handle this, instead of simply using fmprintf, we're using the notifier
in a way can be exposed to users in the proper format and UI.

We were only checking if the prefix of a device name was matching what
we had in the devices list, so if the device list contained "pam", then
also the device "pam-foo" was matching.