When build/rebuilding a site directory, we use a handful of utility

methods to create directories, delete them, and symlink them. These grew

organically, and the names aren't a particularly good fit for some

current and upcoming uses. Some of the code also uses older coding

standards.
This changes up the methods, giving them more general names and adding a

bit more flexibility in their use.
The bulk of link_pkg_dir() is now in mirror_files(), which can be

told to explicitly symlink or explicitly copy a tree, instead of just

using the caller's option. link_pkg_dir() simply wraps it with some

values.
unlink_media_dir() is now remove_files().
The code calling these have been updated to use single quotes and to

avoid repeated attribute lookup.
Support for $sitedir/bin has also been added. This will be created if

it doesn't already exist.

Tested installing and upgrading site directories, and verifying the

resulting directory structure.