This, in addition to 1eb753aa8e5e, fixes "upstream zone" on Windows.

The variable was considered non-existent in the absence of any
valid_referers directives.

Given the following config snippet,

    location / {
        return 200 $invalid_referer;
    }

    location /referer {
        valid_referers server_names;
    }

"location /" should work identically and independently on other
"location /referer".

The fix is to always add the $invalid_referer variable as long
as the module is compiled in, as is done by other modules.

The shared objects should generally be allocated from shared memory.
While peers->name and the data it points to allocated from cf->pool
happened to work on UNIX, it broke on Windows.  On UNIX this worked
only because the shared memory zone for upstreams is re-created for
every new configuration.

But on Windows, a worker process does not inherit the address space
of the master process, so the peers->name pointed to data allocated
from cf->pool by the master process, and was invalid.

No functional changes.

Removed custom variable type and renamed function that adds variables.

The phase is added instead of the try_files phase.  Unlike the old phase, the
new one supports registering multiple handlers.  The try_files implementation is
moved to a separate ngx_http_try_files_module, which now registers a precontent
phase handler.

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.