Fixed alias in regex locations with limit_except/if.

The ngx_http_map_uri_to_path() function used clcf->regex to detect if
it's working within a location given by a regular expression and have
to replace full URI with alias (instead of a part matching the location
prefix).  This is incorrect due to clcf->regex being false in implicit
locations created by if and limit_except.

Fix is to preserve relevant information in clcf->alias instead, by setting
it to NGX_MAX_SIZE_T_VALUE if an alias was specified in a regex location.