Escape any '.' characters in sysctl node names

ZFS creates some sysctl nodes that include a pool name, and '.' is an
allowed character in pool names. But it's the separator in the sysctl
tree, so it can't be included in a sysctl name. Replace it with "%25".
Handily, "%" is illegal in ZFS pool names, so there's no ambiguity
there.

PR: 257316
MFC after: 3 weeks
Sponsored by: Axcient
Reviewed by: freqlabs
Differential Revision: https://reviews.freebsd.org/D31265