Upstream: keep request body file from removal if requested.

Upstream: keep request body file from removal if requested.

The new request flag "preserve_body" indicates that the request body file should
not be removed by the upstream module because it may be used later by a
subrequest.  The flag is set by the SSI (ticket #585), addition and slice
modules.  Additionally, it is also set by the upstream module when a background
cache update subrequest is started to prevent the request body file removal
after an internal redirect.  Only the main request is now allowed to remove the
file.