use path limit in bytes (bb #311)

The real limit depends on nginx's buffer size (large_client_header_buffers),
where the full request line including the verb, volume name, url-encoded path and \0 has to fit in 8k by default.

10 + SXLIMIT_MAX_VOLNAME_LEN + SXLIMIT_MAX_FILENAME_LEN*3 <= large_client_header_buffers