Added fallback detection for TrueNAS systems that may not have
/etc/truenas-version or other standard markers:
1. Check if hostname contains "truenas" (common default hostname)
2. Test if /usr/local/bin is actually writable - if not and /data
exists, use TrueNAS installation paths
This fixes installations on TrueNAS systems where the standard
detection files are missing but the filesystem is still immutable.
Related to #801
774fac9edd