In the 9.17.19 release "tls" statements verification code was
added. The code was too strict and assumed that every such a statement
should have both "cert-file" and "key-file" specified. This turned out
to be a regression, as in some cases we plan to use the "tls"
statement to specify TLS connection parameters.
This commit fixes this behaviour; now a "tls" statement should either
have both "cert-file" and "key-file" specified, or both should be
omitted.