Rename httpd.conf "root strip" option to "request strip".

"root strip" was semantically incorrect and did cause some confusion
as it never stripped the root but the client's request path.

Discussed with many. Heads up: this is a grammar change that also
affects acme-client(1) configurations (see current.html).

OK claudio@

Convert the acme-client test to use a config file acme-client.conf.
Restructure the Makefile, put httpd.conf in separate config file.
Skip test if domain name for letsencrypt.org is not set.
discussed with benno@

Convert the acme-client test to use a config file acme-client.conf.
Restructure the Makefile, put httpd.conf in separate config file.
Skip test if domain name for letsencrypt.org is not set.
discussed with benno@