minor, make work routine part of the ctor params

This means the work routine can't be changed after instantiation.
We currently recommend using a stub function and should changes
be necessary call the real work routine from there.
minor, make the publicly visible params part of the ctx which in turn ...

is the first slot of the private _httpd_s struct.

This is so we just have to pass on one context pointer
and both the derived context fields as well as the
initialisation parameters are visible to our handlers.
minor, pass back filled in www_dir and server in public gand_httpd_t param slot
minor, never destroy the default loop
Merge branch 'core/httpd'

* core/httpd:
  minor, never destroy the default loop
  minor, pass back filled in www_dir and server in public gand_httpd_t param slot
  minor, make the publicly visible params part of the ctx which in turn ...
  minor, make work routine part of the ctor params