Add OpenSSL-compatible alias for ERR_FLAG_STRING flag.

A lot of applications use ERR_TXT_STRING flag in order to determine
whether the error string is printable or not.

Change-Id: Ic79cdb55d43fdfac48da5de5e18c0c9c03c8f747
Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Reviewed-on: https://boringssl-review.googlesource.com/1192
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/err.h b/include/openssl/err.h
index 891fb3e..82f51a6 100644
--- a/include/openssl/err.h
+++ b/include/openssl/err.h
@@ -324,6 +324,9 @@
 /* ERR_FLAG_STRING means that the |data| member is a NUL-terminated string that
  * can be printed. */
 #define ERR_FLAG_STRING 2
+/* ERR_TXT_STRING is provided for compatibility with code that assumes that
+ * it's using OpenSSL. */
+#define ERR_TXT_STRING ERR_FLAG_STRING
 
 /* ERR_FLAG_PUBLIC_MASK is applied to the flags field before it is returned
  * from functions like |ERR_get_error_line_data|. */