barkeep: Add support for remote images

* Add support for http, https and ftp urls provided as an argument to `-c`
* Fix up logic for template checking, now actually check if we have any
  templates at all before testing if the chosen one exists in an
  existing pool
* Add the defacto TMPDIR standard for the tmp directory used by curl
  when downloading an image
libtavern: Make the chk_dirs_exist function more generic, takes multiple arguments.
taverner: Switch to chk_dirs_exist, comment out some debugs and change dmenu colours a bit.