Introduce xbps_binpkg_get_file_into_fd() and use it for xbps-query(8) --cat.

This allows you to print to stdout any file stored in a binary package,
locally or remotely!

$ xbps-query -R --cat=/usr/bin/ls coreutils > ls
$ file ls
ls: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=7a195fc46d1d5cdca32bfccd3b30f81784e342ed, stripped
$