Merge branch 'dt/http-postbuffer-can-be-large'

Allow the http.postbuffer configuration variable to be set to a
size that can be expressed in size_t, which can be larger than
ulong on some platforms.

* dt/http-postbuffer-can-be-large:
  http.postbuffer: allow full range of ssize_t values

diff --cc cache.h
Simple merge

diff --cc http.c
Simple merge

diff --cc remote-curl.c
Simple merge