fix building by bcc without PCRE

named location

dav_access uses ngx_conf_set_access_slot()

proxy_store and fastcgi_store, proxy_store_access and fastcgi_store_access

$is_args

allow to set $args

set worker's perl $$

omit unnecessary code,
escape space, etc in $memcached_key

fix segfault when session was freed twice