Bug 9890: fix plugin handling by koha-create and koha-create-dirs

1) koha-create: fill in __PLUGINS_DIR__
2) koha-create-dirs: create the directory, set permissions (writeable by the koha user)

Sponsored-by: Universidad Nacional de Córdoba
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Robin Sheat <robin@catalyst.net.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>