MINOR: server: implement "from none"

MINOR: server: implement "from none"

Implement "from none" on a server line. This instructs to not preset
server settings via another instance, instead relying on documented
default values.

This is useful to change the default behavior for static servers to
prevent them from reusing a default-server instance. A secondary usage
of this value is on a default-server. This reset the default-server to
the documented default settings.

This is implemented by extending _srv_parse_from() to parse "none"
value. In this case, <from> output return is set to NULL, which will
cause the server to be initialized via srv_settings_init().