feat(mount): add MS_NOSYMFOLLOW to MsFlags

[jzuber4]

What does this PR do

Adds the MS_NOSYMFOLLOW flag to MsFlags in the mount module on Linux and
Android.

MS_NOSYMFOLLOW (bit 0x100) was added to the kernel in Linux 5.10. When
passed to mount(2), it forbids following symbolic links during path
resolution on the mounted filesystem. nix did not expose it, so callers that
need it currently have to reach for unsafe { MsFlags::from_bits_unchecked(0x100) }
and hard-code the raw bit themselves.

The constant is already provided by libc as libc::MS_NOSYMFOLLOW since
0.2.186, which is nix's current minimum libc requirement, so this needs no
dependency bump. The flag is placed in kernel bit order, between MS_DIRSYNC
(0x80) and MS_NOATIME (0x400).

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[jzuber4]

Heads up for reviewers: the red minver check here is not caused by this PR. It's a pre-existing breakage on master (the cfg_aliases 0.2.1 build-dependency floor no longer compiles under -Zdirect-minimal-versions), fixed in #2812. Once #2812 lands I'll rebase this PR onto the updated master and minver will go green.