QUIC: ignore path validation socket error (ticket #2532). Previously, a socket error on a path being validated resulted in validation error and subsequent QUIC connection closure. Now the error is ignored and path validation proceeds as usual, with several retries and a timeout. When validating the old path after an apparent migration, that path may already be unavailable and sendmsg() may return an error, which should not result in QUIC connection close. When validating the new path, it's possible that the new client address is spoofed (See RFC 9000, 9.3.2. On-Path Address Spoofing). This address may as well be unavailable and should not trigger QUIC connection closure.