Hand control to libmilter event loop.
||smfi_main is called after a filter's initialization is complete.
||smfi_main hands control to the Milter event loop.
||smfi_main will return MI_FAILURE if it fails to establish a connection. This may occur for any of a variety of reasons (e.g. invalid address passed to smfi_setconn). The reason for the failure will be logged. Otherwise, smfi_main will return MI_SUCCESS.