apt-file like new 'search-file' command

Hi iMil,

while working on some project, I needed an `apt-file search foo.h` feature but to the best of my knowledge, it is not available for non-installed packages. So I decided to create one for pkgin.

Please note that it needs some support from `pkg_info`. I added a new `-Y` option to it that outputs what is need for the `search-file` command. The modified pkg_info is available here: