Make KiNET output socket non-blocking

[iibaranov-IG]

When a KiNET target disappears, a blocking UDP send can hold OLA's shared event loop while the kernel resolves the unreachable neighbour. That delays unrelated healthy universes and explains the stutter followed by catch-up bursts reported in #1850.

Set the KiNET socket descriptor non-blocking during initialization. If the socket cannot be configured, startup fails explicitly; if a later send cannot proceed, the existing send-failure path drops that frame instead of blocking other outputs.

Validation:

• built the current repository in a clean Debian container with only the KiNET plugin enabled
• plugins/kinet/KiNetTester: all 3 tests pass, including a new check of the actual descriptor's O_NONBLOCK flag
• cpplint passes for both changed files
• BCL evidence: https://github.com/iibaranov-IG/broadcast-control-lab/pull/15
• physical multi-device cadence testing remains requested in the BCL hardware kit

Fixes #1850