Take into account $conf_files and $keep_dirs when removing a pkg.

* If a dir is listed in $keep_dirs it won't be removed (anyway any
  directory that is not empty won't either).
* If a config file hasn't been modified (SHA256 hash matches) the
  file will be removed, otherwise it won't be removed.

There's still missing support to handle new configuration files when
there's an existing file, but will do later.

--HG--
extra : convert_revision : 448af95876fc7a47da07a9a910d6111047f72271

doc/TODO

@@ -11,6 +11,7 @@ Packages:
 
 xbps-bin:
  * Add support to handle conf_files and keep_dirs from package metadata.
+   [PARTIALLY IMPLEMENTED]
  * Add support to detect orphaned packages, something like
    "apt-get autoremove" [IN PROGRESS]
  * Add support to install binary packages without any repository.