Add the users HOME in the global config dict.

BUG:
If the user has set up only one .reviewboardrc in
his/hers home directory, it will not be included in the
search path for configs. It's contents is returned from
load_config_files() and assigned to user_configs in main. How ever
this variable is never used in parse_options().

FIX:
Add the HOME path to the search path for global configs.