rpc: correct resultproc_t's type

It takes exactly three arguments of known type.

Tweak the types of various resultproc_t functions to match the type (mostly
added const to struct pointers) allowing us to drop casts.

Effort: CHERI upstreaming
Reviewed by: vangyzen, glebius
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D54941