libc: add MS_NOSYMFOLLOW support

[cyphar]

Linux platforms that are forced to use the libc backend cannot access MS_NOSYMFOLLOW. The upstream libc crate doesn't expose MS_NOSYMFOLLOW, but on Linux we can just remap linux-raw-sys.

FreeBSD also has nosymfollow support in the form of MNT_NOSYMFOLLOW, but rustix::mount is not provided for FreeBSD.

Signed-off-by: Aleksa Sarai [email protected]