Add functions to calculate approximate disk usage by packages

Add functions to calculate approximate disk usage by packages

Two helper function are added to calculate the disk usage from packages
that are either currently installed on the system or from a package
archive.

Some minor approximations have been made:

1. Size for directories is not considered when removing a package from the
   filesystem to avoid multiple counting across packages. Also, these are
   reported to take zero size while installing.

2. Symlinks are reported to contribute zero size towards removal as
   libarchive reports them to have zero size for install.

3. Package data files (.PKGINFO, .INSTALL, .CHANGELOG) are counted towards
   usage on dbpath on install, but their size is not counted on package
   removal.

4. No handling of extra size needed for .pacsave/.pacnew files.

Signed-off-by: Allan McRae <allan@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>