sys/include: Add utlist to sys includes

This PR adds the [utlist](https://troydhanson.github.io/uthash/utlist.html) header to sys/includes (as proposed in #2159). A fitting license pattern is provided because utlist is made available under a revised 1c-BSD license.

utlist adds a set of macros for basic linked list operations. (It is first used in #2159)
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