openssl: Integrate Peter Wu's SSLKEYLOGFILE implementation

This is an adaptation of 2 of Peter Wu's SSLKEYLOGFILE implementations.

The first one, written for old OpenSSL versions:
https://git.lekensteyn.nl/peter/wireshark-notes/tree/src/sslkeylog.c

The second one, written for BoringSSL and new OpenSSL versions:
https://github.com/curl/curl/pull/1346

Note the first one is GPL licensed but the author gave permission to
waive that license for libcurl.

As of right now this feature is disabled by default, and does not have
a configure option to enable it. To enable this feature define
ENABLE_SSLKEYLOGFILE when building libcurl and set environment
variable SSLKEYLOGFILE to a pathname that will receive the keys.

And in Wireshark change your preferences to point to that key file:
Edit > Preferences > Protocols > SSL > Master-Secret

Co-authored-by: Peter Wu

Ref: https://github.com/curl/curl/pull/1030
Ref: https://github.com/curl/curl/pull/1346

Closes https://github.com/curl/curl/pull/1866

lib/curl_setup.h

@@ -717,6 +717,7 @@ Therefore we specify it explicitly. https://github.com/curl/curl/pull/258
 #if defined(WIN32) || defined(MSDOS)
 #define FOPEN_READTEXT "rt"
 #define FOPEN_WRITETEXT "wt"
+#define FOPEN_APPENDTEXT "at"
 #elif defined(__CYGWIN__)
 /* Cygwin has specific behavior we need to address when WIN32 is not defined.
 https://cygwin.com/cygwin-ug-net/using-textbinary.html
@@ -726,9 +727,11 @@ endings either CRLF or LF so 't' is appropriate.
 */
 #define FOPEN_READTEXT "rt"
 #define FOPEN_WRITETEXT "w"
+#define FOPEN_APPENDTEXT "a"
 #else
 #define FOPEN_READTEXT "r"
 #define FOPEN_WRITETEXT "w"
+#define FOPEN_APPENDTEXT "a"
 #endif
 
 /* WinSock destroys recv() buffer when send() failed.