Make lsof 10x faster by caching /proc/net socket info.

I deliberately didn't do this first time round because for me our lsof
is already 10x faster than traditional lsof, and caching means we potentially
give less information about a socket that's created while we're running.
It turns out that traditional lsof caches anyway, so I guess nobody cares.

This also fixes a mistake where lsof used CFG_FREE instead of CFG_TOYBOX_FREE.
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