devel/libthmap: create port

Concurrent trie-hash map library -- a general purpose associative array,
combining the elements of hashing and radix trie. Highlights:

• Very competitive performance, with logarithmic time complexity on average.
• Lookups are lock-free and inserts/deletes are using fine-grained locking.
• Incremental growth of the data structure (no large resizing/rehashing).
• Optional support for use with shared memory, e.g. memory-mapped file.

The implementation is written in C11 and distributed under the 2-clause BSD
license.

WWW: https://github.com/rmind/thmap

Will be used by npf port.