On macOS, `launchctl bootstrap` of a label still registered in the domain
fails with 5: Input/output error (EIO). That is the *already loaded* case — a
stale registration from an interrupted restart or a bootout that didn't settle
— recoverable by booting the leftover out and bootstrapping again, and distinct
from the domain being genuinely unmanageable.
launchd_install and launchd_start (both bootstrap paths) treated exit 5 as
'launchd cannot manage this macOS version' and silently degraded to a detached
process, losing auto-start at login and crash-restart. Centralize bootstrap in
_launchctl_bootstrap(), which on EIO boots the stale label out and retries once;
only if the retry also fails does the error propagate so callers apply their
existing _launchctl_domain_unsupported fallback for a genuinely broken domain.
launchd_restart already boots out before bootstrapping (its drained job is
almost always still registered, so a plain bootstrap would hit EIO on the common
path), so it keeps its explicit pre-bootout rather than routing through the
bootstrap-first helper. Corrected the stale exit-5 comment that claimed it
always meant an unmanageable domain.
Adds TestLaunchctlBootstrapEioRetry covering clean bootstrap (no bootout),
EIO -> bootout -> retry success, persistent EIO re-raise, and non-EIO re-raise
without a spurious bootout.
8feeb0ccb8