This function returns a positive ID on success; zero on failure.
yaz_connect() prepares for a connection to a Z39.50 target. The zurl argument takes the form host[:port][/database]. If port is omitted 210 is used. If database is omitted Default is used. This function is non-blocking and doesn't attempt to establish a socket - it merely prepares a connect to be performed later when yaz_wait() is called.