Searches a string for the next token delimited by user-specified
       delimiters.

       When the function is called for the first time for a given
       string the init_flag must be 1 and the first token delimited
       by characters in delimiters is returned. In the following calls
       to get the other tokens in the same string, the init_flag must be
       0. The string argument is now ignored because an internal copy is
       now used. The delimiters string may be different from that used
       in the first call. When no more tokens exist, an empty string is
       returned.