fix: skip empty SCM_RIGHTS when no fds to send

[markc]

Summary

• Skip attaching SCM_RIGHTS ancillary data in send_with_fds when the fds slice is empty
• When no fds need to be sent, use sendmsg without ancillary data instead of sending an empty SCM_RIGHTS message

Problem

When fds is empty, send_with_fds constructs an ScmRights(&[]) ancillary message with zero fds. Some EIS servers — notably KWin's org.kde.KWin.EIS.RemoteDesktop — interpret this empty SCM_RIGHTS as malformed and silently stop processing the connection. This causes the client to never receive seat/device events after the handshake completes.

Fix

Check fds.is_empty() before constructing the ancillary buffer. When empty, call sendmsg with an empty SendAncillaryBuffer (no ancillary data at all). When fds are present, behavior is unchanged.

Found while porting ei-type (a keyboard injector for KDE Plasma 6 Wayland) from C/libei to Rust/reis.

[ids1024]

Some EIS servers — notably KWin's org.kde.KWin.EIS.RemoteDesktop — interpret this empty SCM_RIGHTS as malformed and silently stop processing the connection.

Huh, is it not just using libeis, in which case I wouldn't expect KDE to be interacting with this at the socket level in a way that could affect this?

Though it's reasonable enough to do this, particularly since it presumably matches libei.

[markc]

Sorry, I can't remember the exact situation that lead to this PR but the binary I was building would simply not work until this fix was applied.