When we have some amount of a chunk header, but not all of it, we

When we have some amount of a chunk header, but not all of it, we
need to read more from the fd.  The semantics we _really_ want for
that read operation is "wait until at least one char is available,
then return as many as N to us".

This can be done with a combination of system calls, but it is likely
just as cheap to just read one char at a time, so we do that.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1359 d4fa192b-c00b-0410-8231-f00ffab90ce4