Add libconfig import support to remaining conf files

[MrKeiKun]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Adds the import: override mechanism to conf files that were missing it, consistent with the existing pattern used in map-server.conf, char-server.conf, etc.

• conf/atcommand.conf — added import lookup in atcommand_config_read()
• conf/groups.conf — refactored read_config() in pc_groups.c to use a static config pool so all configs stay alive until inheritance resolution and atcommand->load_groups() complete
• conf/channels.conf — split read_channels_config() into a _from_file() helper with recursive import support
• conf/battlegrounds.conf — same wrapper/helper split pattern
• conf/clans.conf — import recurses with reload=false to avoid clearing the DB on the secondary file
• conf/network.conf and conf/api/api_network.conf — added import lookup in socket_net_config_read() (already accepted a filename)
• conf/plugins.conf — replaced the old fopen replace-hack with proper merge support; HPMHooking-first ordering preserved via an is_primary flag

Each modified conf file gets an import: line pointing to conf/import/<name>.conf, and a corresponding skeleton is added to conf/import-tmpl/.

Issues addressed: #62