Add new port: devel/libdistance.

This is a library for calculating the edit distance between strings or data
using various metrics (Levenshtein, Damerau, Needleman-Wunsch, Hamming...).

Note: r400363 used the above commit message but, due to user error, was

actually devel/myrepos (see https://reviews.freebsd.org/D3730).

Reviewed by: garga
Approved by: brooks
Differential Revision: https://reviews.freebsd.org/D4015