x.509/Makefile-{link,server}: Allow users to override FQDNS

By using ?= to assign that variable only if it's not already set [1].
For Makefile-link, also append the 'nginx' entry using += [2], to
ensure it is always present.

[1]: https://www.gnu.org/software/make/manual/html_node/Setting.html
[2]: https://www.gnu.org/software/make/manual/html_node/Appending.html