[tools] Add spell checking scripts

Summary:
Add two small scripts to do spell checking in .po files.

When working with Weblate, the web browser provides some spell checking with
visual hints for every translation entry.

These scripts provide spell checking for the whole file showing the faulting
words in context.

Relies on msgexec(1) and aspell(1) from devel/gettext-tools and
textproc/aspell.

It can be tested with:

./po_spellcheck.sh ../documentation/content/es/articles/bsdl-gpl/_index.po es

Reviewed by: carlavilla@, dbaio@
Differential Revision: https://reviews.freebsd.org/D32465