kern_sysctl: Make name2oid() non-destructive to the name

It is not the first time I see it panicking while trying to modify
const memory. Lets make it safer and easier to use. While there,
mark few functions using it also const.

MFC after: 10 days

(cherry picked from commit f80babf906b7be51b2a031ef26525893c7bf4e31)