Add y_get_cleansed_parameter(), y_cleanse_parameter_ext(), add checks.

Housecleaning:
- Add y_get_cleansed_parameter(), y_cleanse_parameter_ext()
- Make y_left use y_strdup()
- Make y_uppercase_parameter() check for the existance of said parameter.
- Add checks for empty/nonexistant parameters to all of the above.