Add helper functions to update the target pkg SQLite database when NanoBSD modifies configuration files and filesystem layout:

• Refresh config_files content and sha256 checksums when etc/defaults/rc.conf, etc/ttys, or etc/ssh/sshd_config are modified.
• Update files table paths from /usr/local/etc to /etc/local.
• Repoint /tmp rows to /var/tmp in pkg_directories after /tmp is symlinked.
• Set file mtimes and package times to NANO_TIMESTAMP for reproducible builds.