LinuxKPI: add LINUXKPI_PARAM_charp()

Add yet another version of the various module_param_named() use cases.
This one deals with "charp".

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30433

(cherry picked from commit c1661d59e68e29a817c870f421a6871563039dbb)