Changes for 1.99

[seritools]

• Rebase once the beta is out
• Verify Stabilize fs_set_times rust-lang/rust#160820 works/is correct on r9x
• Go through commits and squash later fixes to the same areas as earlier ones
• Verify dirfd file operations (2/4) rust-lang/rust#150679 -- all good, creating Dir fails on systems that don't support it