distro-build: fix Windows client setup
config: add `doc-open-url` configuration

This configuration option is a better way to redirect documentation
access (at a place like NEU) than patching "search.rkt".
setup/unixstyle-install: preserve all other config entries

Preserve any configuration entry that is not explicitly set,
as opposed to keeping only entries that are known to the
script.
raco pkg config: add `doc-open-url` key