Small patches for pkg_add

I've written a small patch for pkg_install that removes the system call to tar
in pkg_add command replacing it by some libarchive code.

I'm planning to do some cleanup by time to time sending small patches on
pkg_install. My very first goal is to remove most of the system call, and to
understand the way the code works, not to rewrite everything.

Here is the first patch :
- modification of the unpack function to use libarchive to extract data
- it does not modify the way things works: it does exactly the same thing
  without the system call
- I also removed two unused function from lib/file.c

It is my first patch for src, I don't know the good process for that.
Hope to send more patches soon.

This is not another attempt to rewrite pkgtools, I just want to improve them the
time that people working on the different rewrites finish their work.

the patch is join to this mail, or can be found at the following link:
http://etoilebsd.net/~bapt/pkgtools-libarchive-patch1.patch

regards
Bapt