We can't (apparently) only rely on the return value of start_tls(), we have to check the value of the crypt session too.
This fix the possibility of this happening:
Jun  7 18:07:26 sbbs synchronet: ftp  0058 TLS ERROR 'No permiss.to perform this operation' (-21) opening keyset
Jun  7 18:07:26 sbbs synchronet: ftp  0058 <192.168.1.25> initialized TLS successfully

Instead, we'll detect the failure and disable FTPS support, logging "failed to initialize TLS successfully".