civetweb support mbedtls

[Qinch]

#839
The Pull Request without test.
I will try to compile and test the code, In the next few days.

As I said before,
the demo using compile option -DUSE_MBEDTLS and -DNO_SSL at the same time. NO_SSL is used to compile without OpenSSL. USE_MBEDTLS means that mbed TLS is compiled.

[codecov]

Codecov Report

Merging #912 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #912   +/-   ##
=======================================
  Coverage   77.17%   77.17%           
=======================================
  Files          13       13           
  Lines       10255    10255           
=======================================
  Hits         7914     7914           
  Misses       2341     2341           

Impacted Files	Coverage Δ
src/civetweb.c	68.92% <ø> (ø)

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[bel2125]

Merged to branch "mbedtls": https://github.com/civetweb/civetweb/tree/mbedtls

[Qinch]

@bel2125 I will compile the mbedtls branch. and provide a guidance document(such as docs/mbedtls.md).

note that:civetweb need to use Arm Mbed TLS in threaded environments.
see:
https://tls.mbed.org/kb/development/thread-safety-and-multi-threading)