Enhance OpenBSD pkg.conf handling

This moves the pkg.conf parsing into it's own method which allows for better
re-use. While here, support for parsing '+=' lines has been added. This allows
for appending to PKG_PATH, instead of merely setting it with '='.

(#21930) Enchance OpenBSD pkg.conf handling
Merge pull request #1798 from jasperla/pkgconf

Enchance OpenBSD pkg.conf handling