Cache: fixed minimum cache keys zone size limit.

Size of a shared memory zones must be at least two pages - one page
for slab allocator internal data, and another page for actual allocations.
Using 8192 instead is wrong, as there are systems with page sizes other
than 4096.

Note well that two pages is usually too low as well.  In particular, cache
is likely to use two allocations of different sizes for global structures,
and at least four pages will be needed to properly allocate cache nodes.
Except in a few very special cases, with keys zone of just two pages nginx
won't be able to start.  Other uses of shared memory impose a limit
of 8 pages, which provides some room for global allocations.  This patch
doesn't try to address this though.

Inspired by ticket #1665.

src/http/ngx_http_file_cache.c

@@ -2427,7 +2427,7 @@ ngx_http_file_cache_set_slot(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
                 s.data = p;
 
                 size = ngx_parse_size(&s);
-                if (size > 8191) {
+                if (size >= (ssize_t) (2 * ngx_pagesize)) {
                     continue;
                 }
             }