config.yaml now holds only the global section (log-level, unshare-*,
with-veth, with-ipv6); a new persistent.yaml holds volumes/networks.
load_config_file()/write_config_file() are replaced by
load_global_config()/load_persistent_config()/write_global_config()/
write_persistent_config(), each touching only their own file.
-c/--config-file <path> lets one invocation use an alternate file for the
global section only -- persistent.yaml is always the one fixed path,
regardless of -c, so an experiment can never affect real volumes/networks
(a -c file's own volumes/networks, if any, are simply never read either).
A -c path that doesn't exist is a hard error, unlike the default path's
existing missing-file leniency.
migrate_legacy_config_if_needed() moves volumes/networks out of an
old-format config.yaml into persistent.yaml on first run after upgrading,
always against the fixed default paths regardless of -c. A name collision
aborts the migration for that run (touching neither file) rather than
risking data loss.
Required reordering main() to parse CLI args before loading config (so
-c's value is known first) -- ParsedArgs::log_level_flag_given tracks
whether --log-level was already given so the config file's own log-level
doesn't clobber it despite the reversed call order.
ceamac
cbec986e78