Add support for Strict/Mutual TLS to dig

Add support for Strict/Mutual TLS to dig

This commit adds support for Strict/Mutual TLS to dig.

The new command-line options and their behaviour are modelled after
kdig (+tls-ca, +tls-hostname, +tls-certfile, +tls-keyfile) for
compatibility reasons. That is, using +tls-* is sufficient to enable
DoT in dig, implying +tls-ca

If there is no other DNS transport specified via command-line,
specifying any of +tls-* options makes dig use DoT. In this case, its
behaviour is the same as if +tls-ca is specified: that is, the remote
peer's certificate is verified using the platform-specific
intermediate CA certificates store. This behaviour is introduced for
compatibility with kdig.