rpc: correct resultproc_t's type

[brooks]

Summary

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
Sponsored by: DARPA, AFRL

[ngie]

@brooks : not sure if my tree is out of date, but I found a few more references of potential concern:

% rg -l '\(resultproc_t\)' worktree/main/ | sort | sed -e 's,worktree/main/,,g'
...
usr.bin/rup/rup.c
usr.bin/rusers/rusers.c
usr.sbin/bootparamd/callbootd/callbootd.c
usr.sbin/ypbind/ypbind.c

Do those need to be changed?

[ngie]

In D54941#1256407, @ngie wrote:

...

Apart from that--the change looks good to go! Not spotting any obvious landmines in our $work fork either that would be hit by this change.

CC: @vangyzen

[brooks]

In D54941#1256407, @ngie wrote:

@brooks : not sure if my tree is out of date, but I found a few more references of potential concern:

% rg -l '\(resultproc_t\)' worktree/main/ | sort | sed -e 's,worktree/main/,,g'
...
usr.bin/rup/rup.c
usr.bin/rusers/rusers.c
usr.sbin/bootparamd/callbootd/callbootd.c
usr.sbin/ypbind/ypbind.c

Do those need to be changed?

Those are removed in D54940.

[brooks]

rebase