Content generator functions now take arguments describing the buffer
they are to fill, and respond with a struct containing the number of
bytes filled, and the number of free bytes needed before being called
again.
The HTTP response logic now fills the buffer as much as possible before
calling write(2) by topping it up instead of waiting for it to be
completely emptied before generating more content.
Support generated content with an unspecified Content-Length. Generator
functions return 1 if there is more content to come, and 0 if they have
just produced the last piece of content.
So that "http_server.h" does not have to include "serval.h" which
creates a circular dependency.
Remove the __SERVALDNA__HTTP_SERVER_IMPLEMENTATION hack from
"http_server.h"
