Update NA matching support in joins

[hadley]

Waiting on r-lib/vctrs#568

And deprecate use of pkgconfig, as I'm now confident that the current behaviour is the correct default for R.

[hadley]

No use of pkgconfig + na_matches in CRAN: https://github.com/search?q=org%3Acran+na_matches&type=Code

Looks like only two people used it on Github R code generally: https://github.com/search?l=R&p=9&q=na_matches&type=Code