install: add --with-httpd-conf-file to allow specific apache config to be installed

fixes #3962
install: make install-webconf should install htpasswd.users automatically, allowing default icingaadmin login out-of-the-box

fixes #3963
install: add --enable-classicui-standalone

refs #3869
install: add 'make classicui-standalone' and 'make install-classicui-standalone'

refs #3963
classic ui: fix logic on "standalone_installation" setting (if set, ignore icinga.cfg but use cgi.cfg)

refs #3869
classic ui: clarify a bit on cgi.cfg what standalone install means

refs #3869
docs: add standalone_installation