Handle username and password empty submit and correctly call theme function to prevent showing empty dialog with no content

Make check if logo exist use a relative file path, this is better for compatibility of shared hosting environments

Fix page preprocess function to check if variables exist, preventing notices and if logo file exists

Rework template output to re-use core preprocess and process functions as well as revamp tpl to be a full html document and include site logo in output

Added Variable module integration.

#1414678 by Yura and Patrrizio: Fixed undefined variable.
#1011890 by Deciphered: Additional Drupal 7 port issues.
by Deciphered: Fixed coder issues.

Conflicts:
	securesite.inc

  Sites which had disabled authentication would have authentication forced when
  updating.