cli,sdnotify: improve the handling of unix socket directories

[cockroach-teamcity]

Exalate commented:

Related PR: cockroachdb/cockroach#84532
Commit: cockroachdb/cockroach@265667c

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Release note (cli change): CockroachDB now produces a clearer error
when the path specified via --socket-dir is too long.

Release note (cli change): When the flag --background is specified,
CockroachDB now makes 3 attempts to find a suitable directory to
create the notification socket: the value of --socket-dir if
specified, the value of $TMPDIR (or /tmp if the env var is empty),
and the current working directory. If none of these directories has a
name short enough, an explanatory error is printed.

Jira Issue: DOC-5123