portability: fix build on Solaris-derived systemd Current Illumos and older Solaris require _XOPEN_SOURCE for msg_control. O_DIRECTORY doesn't exist on older systems either, so fallback to O_RDONLY. It's good enough as a repository will require both R and X permission anyway.

mmap: only use mmap to read revlog persistent nodemap if it is safe Cf `is_mmap_safe` docstring.