Add a function that compares two etags that might be of different types.
If any of them is weak, the 'W/' prefix is discarded and a strict string
comparison is performed.
Co-authored-by: Tim Duesterhus <tim@bastelstu.be>
Directly including stddef.h in many files results in it being processed
multiple times while it can be centralized in api-t.h and be guarded
against multiple inclusions. Doing so reduces the number of preprocessed
lines by 1200!
So the enums and structs were placed into http-t.h and the functions
into http.h. This revealed that several files were dependeng on http.h
but not including it, as it was silently inherited via other files.
