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>