allow Pre-Install-Pkgs hooks to get info over an FD != stdin

This adds ::InfoFD option alongside the ::Version one to request sending
the information to the specified FD, by default it is STDIN as it was
the case before.

The environment variable APT_HOOK_INFO_FD contains the FD the data is on as
a confirmation that the APT version used understood the request.

Allowing the hook to choose the FD is needed/helpful e.g. for shellscripts
which have a hard time accessing FDs above 9 (as >= 10 are usually used
 internally by them)

Closes: #671726