If an emulation uses those functions instead of set_config_value_node
or set_config_value, it allows the config values to get
overwritten. Introducing new functions is much more readable than
if else statements in the emulation code.

Reviewed by:	khng
MFC after:	2 weeks
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D33770