Feature/diffpkg output modes

The option switch case only matches by splitting via '|' instead of ','

On certain packaging machines where the pacman cache gets updated very
infrequently, the behavior of diffpkg may not function correctly as old
packages were to be downloaded as diff target. In such cases we look for
a pool directory first and search via a glob for an available pool
package sorted by version.

The pool search glob has three glob segments each disallowing the dash
delimiter to split across pkgrel, pkgver and arch. This will return the
correct package from the pool without considering overly eager wildcards
that potentially match different pkgnames.

The default pool search directory is /srv/ftp/pool

The magic values `columns` and `auto` allow to set specific aspects,
with 'auto' as the default value:

- auto: Set width to the maximum line length of all input files
- columns: Set width to the shell defined $COLUMNS env var

Furthermore any number can be passed to set a static width.