This adds the libqb_http API, which is designed to support HTTP and
HTTPS usage from QB64-PE source.
The design consists of a single thread which services all the HTTP(s)
connections. There are then various libqb_http APIs exposed that allow
interacting with this thread to create a new connection, query
connection status, read data, or close the connection.
Internally the thread makes use of the curl_multi interface to allow a
single thread to service multiple HTTP(s) connections in parallel. This
means you can _OPENCLIENT() multiple HTTP connection in a row and all of
them will be serviced at the same time in whatever order data is
available.
HTTP is optional and selected via a Makefile setting. A stub is swapped
in if HTTP support is not used, which avoids need to add another build
flag to libqb.cpp.
