Add "protocols" options to the "tls" clause

Add "protocols" options to the "tls" clause

This commit adds the ability to specify allowed TLS protocols versions
within the "tls" clause. If an unsupported TLS protocol version is
specified in a file, the configuration file will not pass
verification.

Also, this commit adds strict checks for "tls" clauses verification,
in particular:

- it ensures that loading configuration files containing duplicated
"tls" clauses is not allowed;

- it ensures that loading configuration files containing "tls" clauses
missing "cert-file" or "key-file" is not allowed;

- it ensures that loading configuration files containing "tls" clauses
named as "ephemeral" or "none" is not allowed.