net: sockets: tls: Add missing symbols for a few options

`TLS_PEER_VERIFY` and `TLS_DTLS_ROLE` options accept specific values,
yet no symbols were defined for them. In result, magic numbers were used
in several places, making the code less readable.

Fix this issue, by adding the missing symbols to the `socket.h` header,
and using them in places where related socket options are set.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>