openssl: remove legacy cruft, document macro guards

openssl: remove legacy cruft, document macro guards

- assume:
  - `BIO_CTRL_EOF`
  - `SSL_CTRL_SET_MSG_CALLBACK`
  - `SSL_CTRL_SET_MSG_CALLBACK`
  - `SSL_CTRL_SET_TLSEXT_HOSTNAME`
  - `SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER`
  - `SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS`
  - `SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG`
  - `SSL_OP_NO_COMPRESSION`
  - `SSL_OP_NO_TICKET`
  - `X509_V_FLAG_PARTIAL_CHAIN`
  - `X509_V_FLAG_TRUSTED_FIRST`
  They are present in all supported OpenSSL (and fork) versions.

- replace `SSL_ERROR_WANT_EARLY` with `SSL_ERROR_WANT_CLIENT_HELLO_CB`.
  The former appeared in OpenSSL 1.1.1-dev, but renamed before
  the stable release.

- document support for macros:
  - `ENGINE_CTRL_GET_CMD_FROM_NAME`
  - `SSL_ERROR_WANT_ASYNC_JOB`
  - `SSL_ERROR_WANT_ASYNC`
  - `SSL2_VERSION_MAJOR`
  - `TLS1_3_VERSION`

- drop legacy fallback for `CONF_MFLAGS_DEFAULT_SECTION`.
  It was there for OpenSSL 0.9.8 support.

- fix `SSL_CTRL_SET_MSG_CALLBACK` accidentally serving as a guard for
  OpenSSL (and forks) as a whole.

Tested OK with OpenSSL 1.0.2 and 1.1.0 in CI.

Closes #18351