config: new `iproto.ssl` section

A new iproto.ssl section has been introduced. It has the following
options.

• iproto.ssl

SSL parameters required for encrypted connections. These parameters
would be used to set up SSL IProto sockets and to connect to other
instances which require certificate authority (CA).

• iproto.ssl.ca_file

(Optional) A path to a trusted certificate authorities (CA) file. If not
set, the peer won't be checked for authenticity.

Both a server and a client can use the ca_file parameter:

• If it's on the server side, the server verifies the client.
• If it's on the client side, the client verifies the server.
• If both sides have the CA files, the server and the client verify each
  other.

• iproto.ssl.ssl_cert

A path to an SSL certificate file:

• For a server, it's mandatory.
• For a client, it's mandatory if the ca_file parameter is set for a
  server; otherwise, optional.

• iproto.ssl.ssl_ciphers (Optional) A colon-separated (:) list of SSL
  cipher suites the connection can use. Note that the list is not
  validated: if a cipher suite is unknown, Tarantool ignores it, doesn't
  establish the connection, and writes to the log that no shared cipher
  was found.

• iproto.ssl.ssl_key A path to a private SSL key file:

• For a server, it's mandatory.
• For a client, it's mandatory if the ca_file parameter is set for a
  server; otherwise, optional.

If the private key is encrypted, provide a password for it in the
ssl_password or ssl_password_file parameter

• iproto.ssl.ssl_password

(Optional) A password for an encrypted private SSL key provided using
ssl_key. Alternatively, the password can be provided in
ssl_password_file.

Tarantool applies the ssl_password and ssl_password_file parameters
in the following order:

• If ssl_password is provided, Tarantool tries to decrypt the private
  key with it.
• If ssl_password is incorrect or isn't provided, Tarantool tries all
  passwords from ssl_password_file one by one in the order they are
  written.
• If ssl_password and all passwords from ssl_password_file are
  incorrect, or none of them is provided, Tarantool treats the private
  key as unencrypted.

• iproto.ssl.ssl_password_file (Optional) A text file with one or more
  passwords for encrypted private SSL keys provided using ssl_key
  (each on a separate line). Alternatively, the password can be provided
  in ssl_password.

This section is a replacement for the parameters
iproto.listen.*.params.ssl_* and iproto.advertise.*.params.ssl_*.

The hints that redirect iproto.listen.*.params.ssl_* and
iproto.advertise.*.params.ssl_* to <uri>.params.ssl_* should be
marked as ones overwriting iproto.ssl options and marked for advanced
use only with a hint that the user is likely want to use iproto.ssl.
Also, SSL Tarantool examples should be changed w.r.t. new section
iproto.ssl instead.
Requested by @georgiy-belyanin in https://github.com/tarantool/tarantool/commit/a54a406dd26a2ccd3072c37dca0547214c819894.

Locate the configuration reference for iproto.listen..params.ssl_ and iproto.advertise..params.ssl_, along with the SSL Tarantool examples. Update them to use iproto.ssl, mark the legacy hints as overwriting and advanced-only, and preserve the documented option behavior and migration guidance.