Add function to pass libmicrohttpd options when starting Prometheus listener

[sysvinit]

This commit adds a new function, promhttp_start_daemon_with_options(), which exposes the variadic options interface of MHD_start_daemon() to applications using prometheus-client-c. This would allow applications to e.g. bind to a specific IP address instead of the wildcard address, which is the libmicrohttpd default.

[ggarber]

I think this is a great addition. I also need it!

[wferi]

#79 enables a different approach: calling libmicrohttpd directly.