stop using thread to handle HTTP request

LibHTTPD wasn't supported multi-threading at all.  Some static variable,
for example `buf` in `httpdReadRequest`, were shared by all threads.

These cause a lot of unexpected behaviors and sgefaults at first.
However there are also some patches trying to solve it:
  http://dev.wifidog.org/ticket/471 .

Those patches reduce the chance that cause the NoDogSplash crash, but
this bug is still exist.

There are two ways to solve it.  One is remove the `static` modifier and
it brings more overhead of each request.  Another solution is don't use
threads at all.

I prefer the later one.  Since all requests can be handled very quickly,
I beleive that won't increase the response time even if lot of users
trying to access it at the same time.