Add new port devel/mtbl.

mtbl is a C library implementation of the Sorted String Table
(SSTable) data structure, based on the SSTable implementation in
the open source Google LevelDB library https://github.com/google/leveldb.
An SSTable is a file containing an immutable mapping of keys to
values. Keys are stored in sorted order, with an index at the end
of the file allowing keys to be located quickly.

mtbl is not a database library. It does not provide an updateable
key-value data store, but rather exposes primitives for creating,
searching and merging SSTable files. Unlike databases which use the
SSTable data structure internally as part of their data store,
management of SSTable files -- creation, merging, deletion, combining
of search results from multiple SSTables -- is left to the discretion
of the mtbl library user.

Differential Revision: https://reviews.freebsd.org/D2206
Approved by: mat (mentor)
Sponsored by: Farsight Security, Inc.