[FS#1285] libcurl with mbedtls async interface doesn't work

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Darren Tucker (dtucker) 

Attached to Project - OpenWrt/LEDE Project
Summary - libcurl with mbedtls async interface doesn't work
Task Type - Bug Report
Category - Packages
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - lede-17.01
Due in Version - Undecided
Due Date - Undecided
Details - Supply the following if possible:
 - Device problem occurs on: GL.inet AR150 (but not device specific).
 - Software versions of OpenWrt/LEDE release, packages, etc: lede-17.01 branch, libcurl-7.52.1-6.
 - Steps to reproduce:  install https_dns_client package and try to use it.  You will see all requests failing with "CURLINFO_RESPONSE_CODE: 0", suspiciously quick server times and a result "buflen 0".

>From my comment in https://github.com/aarond10/https_dns_proxy/issues/9
"""
The OpenWRT/LEDE problem is in libcurl.

On LEDE 17.01.2 with all updates, after annotating the callbacks with some logging and running it under strace I get the following debug log:

socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 6
[...]
connect(6, {sa_family=AF_INET, sin_port=htons(443), sin_addr=inet_addr("172.217.160.46")}, 16) = -1 EINPROGRESS (Operation in progress)
[..]
https_client.c:242 multi_sock_cb called, sock 6 what 2 # ie CURL_POLL_OUT
[...]
https_client.c:214 sock_cb called, fd 6 revents 2 # SSL socket gets written to
https_client.c:242 multi_sock_cb called, sock 6 what 4 # ie CURL_POLL_REMOVE

ie as soon as the socket gets written to, libcurl tells the app to stop monitoring the connection to the server and retrieve the results, which unsurprisingly are empty ("CURLINFO_RESPONSE_CODE: 0", suspiciously quick server times and a result "buflen 0").

This seems to be a known problem in libcurl