Impl syslog::setlogmask

[tisonkun]

This follows up #2537 and refers to #413.

cc @SteveLauC

[SteveLauC]

Thanks!

Regarding those 2 syslog C macros, I am working on sending them to the libc crate, let's merge this PR first.

[tisonkun]

@SteveLauC May we cut a release recently?

[SteveLauC]

Honestly, hard to tell, I plan to maintain open-source projects for only 7 hours per week, and there are still some I/O safety leftovers to clean, if we are lucky, we will have a release in 2 or 3 weeks

[tisonkun]

@SteveLauC Good to know. Do you have a list of the release blockers? Or we have a label/milestone/tracking issue to list them. I may try to see if anything I can help.

[SteveLauC]

No detailed to-do lists, grep RawFd in the codebase, and if a raw fd can be replaced with I/O-safe fd types (OwnedFd, BorrowedFd), do it. Here is an example PR.

I think 90% of interfaces are already I/O-safe, the leftovers are harder to do, e.g., the ScmRights control message, original I thought the interface included in that PR should work just fine, then I realized you could send file descriptors within the same process (kinda meaningless, but nothing stops you from doing so), in this case, my interface could lead to double-close quite easily.

Once this is done, do a crates-test, if we still have time:)