Add FreeBSD/OpenBSD support.

[lcook]

Since there's no definition for detectors for either [Free/Open]BSD it'll fail to build:

(lcook@darkstar:go-locale) (git)[master:2bb2ed4]
% uname -a
FreeBSD darkstar 12.1-RELEASE-p3 FreeBSD 12.1-RELEASE-p3 GENERIC  amd64
(lcook@darkstar:go-locale) (git)[master:2bb2ed4]
% go build
# github.com/Xuanwo/go-locale
./locale.go:40:21: undefined: detectors

Adding both locale_freebsd and locale_openbsd provides that definition and subsequently fixes this.

[codecov]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           31        31           
=========================================
  Hits            31        31           

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[Xuanwo]

LGTM!