On Windows, a worker process does not call ngx_slab_init() from
ngx_init_zone_pool(), so ngx_slab_max_size, ngx_slab_exact_size,
and ngx_slab_exact_shift were left uninitialized.

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.