TLS: Provide ESNI support framework for curl and libcurl

ESNI-FRAMEWORK-README.md

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+# TLS: Provide ESNI support framework for curl and libcurl
+
+The proposed change provides a framework to facilitate work to
+implement ESNI support in curl and libcurl. It is not intended
+either to provide ESNI functionality or to favour any particular
+TLS-providing backend. Specifically, the change reserves a
+feature bit for ESNI support (symbol `CURL_VERSION_ESNI`),
+implements setting and reporting of this bit, includes dummy
+book-keeping for the symbol, adds a build-time configuration
+option (`--enable-esni`), provides an extensible check for
+resources available to provide ESNI support, and defines a
+compiler pre-processor symbol (`USE_ESNI`) accordingly.
+
+Proposed-by: @niallor (Niall O'Reilly)
+Encouraged-by: @sftcd (Stephen Farrell)
+See-also: [this message](https://curl.haxx.se/mail/lib-2019-05/0108.html)
+Limitations:
+-   Book-keeping (symbols-in-versions) needs real release number, not 'DUMMY'.
+
+-   Framework is incomplete, as it covers autoconf, but not cmake.
+
+-   Check for available resources, although extensible, refers only to
+    specific work in progress ([described
+    here](https://github.com/sftcd/openssl/tree/master/esnistuff)) to
+    implement ESNI for OpenSSL, as this is the immediate motivation
+    for the proposed change.