This condition caused aborting BBS operations via Ctrl-C to fail when
connecting from a traditional Telnet client using binary transmission
mode (e.g. telnet -8) or SyncTERM v1.2rc6.

The check of rio_abortable was probably enough to disable this function
during file transfers (e.g. we've had no issues with SSH file transfers
that might include byte 0x03 received from the client), but I added the
check for SS_FILEXFER system status check too, perhaps unnecessary.