Rewrite supply tracing code.

Two passes.

First a quick breadth first forward scan to find hexes
that can be reached from the supply source at all.

Second an exhaustive recursive search from each unit to
find the actual used supply lines and hexes. This uses the
results from the first search to quickly prune impossible
paths.

This also fixes a bug where some supply chains from
highways via mixed highway, track, and trail to another
bit of highway were missed.

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