Makefile: add install and uninstall targets

Pierre Proncheryrequested to merge
khorben/bsddialog:khorben/install-target into main

This applies to the BSD and GNU Makefiles, in two separate commits; feel free to squash them into one if you accept this merge request. While there, I registered phony rules as such.

Tested on NetBSD/amd64.

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