Support for Domain Access module (Variable module)

Attached untested patch:

• replaces some instances of variable_get() with variable_get_value() for the Flickr main module (see below)
• provides an update function to delete all unused variables on running update.php and removes them from the uninstall hook
• defines the variables as domain_variables.

Only affects the following variables (machine names):

• flickr_default_userid
• flickr_photos_per_page
• flickr_title_suppress_on_small
• flickr_metadata_suppress_on_small
• flickr_info_overlay
• flickr_license

Correction. Remains untested.

Removed one variable too much:

-  variable_del('flickr_caption_padding');