Replace /usr/local with /opt/local

This replaces /usr/local with /opt/local as the override directory, and adds boot-time kernel cmdline controls to manage its contents before they can integrate into the running system.

/usr/local is no longer used. All local overrides are now placed under /opt/local instead.

A kernel cmdline parameter is introduced to control /opt/local at boot:

kde-linux.mask_local=1 — Masks /opt/local with a tmpfs early in boot, preventing any of its contents from being picked up by the system. The real content remains accessible at /var/opt/local via a private bind mount, allowing inspection and modification without system integration.

Signed-off-by: Hadi Chokr hadichokr@icloud.com