runq: Tidy up and rename runq_setbit() and runq_clrbit()

Factorize common sub-expressions in a separate helper (runq_sw_apply())
for better readability.

Rename these functions so that the names refer to the use cases rather
than the implementations.

Reviewed by: kib
MFC after: 1 month
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45387

(cherry picked from commit 757bab06fb59abca8b7fda38e8f546ff7c2d3280)