libc should have had a function like this at least 25 years ago :-/

the calling code is responsible for poll(2)'ing the fd for
connection completion.

be used outside vss.c

callback function.

size limitation, the request fails and the connection is closed.

Fixes #1664

specific wait/exit code.

This is a compromise discussed at VDD15Q1 after a previous suggestion
to expose the 304 response status directly to VCL.

This way, VCL can differenciate between 200 and 304 responses, but simpler
processing logic just checking for status 200 can be preserved where
differenciating is not required.

A few things still remaining.