Internal helper function for checking specific arguments used in custom_search.
check_args_flag(flag, negate)A string specifying the flag to be used for filtering messages. Use flag_options to list the flags available in your IMAP server.
If TRUE, negates the search and seeks for
"NOT search_criterion". Default is FALSE.
NULL if arguments are correct.
Other customsearch checkargs functions: check_args_date,
check_args_size,
check_args_string,
check_args_within