[Fix] Reduce control message size to prevent sendmsg crash

[Fix] Reduce control message size to prevent sendmsg crash

The rspamd_srv_command and rspamd_control_command structures grew too
large (~8KB) due to multiple CONTROL_PATHLEN fields in mp_loaded and
re_map_loaded, exceeding socket buffer limits and causing crashes in
sendmsg during worker startup.

Fix by:
- Removing redundant cache_dir fields (all processes know it from config)
- Using consistent name[64] for both mp_loaded and re_map_loaded
- Getting cache_dir from cfg->hs_cache_dir at receive time instead