Introduce QUADLET_UNIT_PATH (similar to SYSTEMD_UNIT_PATH)

[m0nhawk]

Introduce a documented, opt-in environment variable similar to SYSTEMD_UNIT_PATH:

QUADLET_UNIT_PATH=/dir/one:/dir/two:/dir/three

With behavior similar to SYSTEMD_UNIT_PATH:

When the variable $SYSTEMD_UNIT_PATH is set, the contents of this variable overrides the unit load path. If $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.

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I can try to draft a PR, but I will definitely have a lot of question as I am not familiar with podman organization and code.