rust: use `cpython` 0.7.2 crate to add support for Python 3.12 This will give us more headroom until we can migrate to PyO3 some day.

revbranchcache: disable mmap access by default The revbranchcache can be truncated (if some part of it is detected as invalid). Using mmap on file we truncate is not an option at access to truncated part would result in a SIGBUS signal. So we disable the mmap by default until we fix this issue.

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.