kqueue: use from_bits_retain to avoid panics on unknown flags

[kamalmarhubi]

What does this PR do

KEvent::flags() and KEvent::fflags() used from_bits().unwrap(), which panics if the kernel returns flags unknown to the current bitflags definition. Use from_bits_retain to preserve all bits without panicking.

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

[xtqqczze]

from_bits_retain only became available with #2027, so it's less likely the original intent was to panic.

[kamalmarhubi]

from_bits_retain only became available with #2027, so it's less likely the original intent was to panic.

Ah thanks for that context! I just assumed it was originally done in error since the in theory a newer kernel could return bits we don't know about.