DISPATCH-1513: add LWS flag to allow plain HTTP on an HTTPS listener.

[mgoulish]

One line fix -- we just need to tell Libwebsockets that we want it to do this, and it will let us.

[ganeshmurthy]

Compiling this on Fedora 29 gives the following error -
/home/gmurthy/opensource/qpid-dispatch/src/http-libwebsockets.c: In function ‘listener_start’:
/home/gmurthy/opensource/qpid-dispatch/src/http-libwebsockets.c:357:87: error: ‘LWS_SERVER_OPTION_ALLOW_HTTP_ON_HTTPS_LISTENER’ undeclared (first use in this function); did you mean ‘LWS_SERVER_OPTION_ALLOW_LISTEN_SHARE’?
(config->ssl_required ? 0 : LWS_SERVER_OPTION_ALLOW_NON_SSL_ON_SSL_PORT | LWS_SERVER_OPTION_ALLOW_HTTP_ON_HTTPS_LISTENER ) |
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
LWS_SERVER_OPTION_ALLOW_LISTEN_SHARE
/home/gmurthy/opensource/qpid-dispatch/src/http-libwebsockets.c:357:87: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [src/CMakeFiles/qpid-dispatch.dir/build.make:978: src/CMakeFiles/qpid-dispatch.dir/http-libwebsockets.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:987: src/CMakeFiles/qpid-dispatch.dir/all] Error 2
make: *** [Makefile:141: all] Error 2
[gmurthy@localhost build]$

Looks like the flag LWS_SERVER_OPTION_ALLOW_HTTP_ON_HTTPS_LISTENER was introduced in libwebsockets 3.2.0 ?

[mgoulish]

Fixed on master.