url: add CURLOPT_SSL_FALSESTART option

This option can be used to enable/disable TLS False Start defined in the RFC
draft-bmoeller-tls-falsestart.

lib/vtls/vtls.c

@@ -857,4 +857,16 @@ bool Curl_ssl_cert_status_request(void)
 #endif
 }
 
+/*
+ * Check whether the SSL backend supports false start.
+ */
+bool Curl_ssl_false_start(void)
+{
+#ifdef curlssl_false_start
+  return curlssl_false_start();
+#else
+  return FALSE;
+#endif
+}
+
 #endif /* USE_SSL */

lib/vtls/vtls.h

@@ -118,6 +118,8 @@ CURLcode Curl_pin_peer_pubkey(const char *pinnedpubkey,
 
 bool Curl_ssl_cert_status_request(void);
 
+bool Curl_ssl_false_start(void);
+
 #define SSL_SHUTDOWN_TIMEOUT 10000 /* ms */
 
 #else
@@ -145,6 +147,7 @@ bool Curl_ssl_cert_status_request(void);
 #define Curl_ssl_kill_session(x) Curl_nop_stmt
 #define Curl_ssl_random(x,y,z) ((void)x, CURLE_NOT_BUILT_IN)
 #define Curl_ssl_cert_status_request() FALSE
+#define Curl_ssl_false_start() FALSE
 #endif
 
 #endif /* HEADER_CURL_VTLS_H */