Cache the OS split-DNS capability while compiling DNS config and return directly for split-capable platforms that do not need the Apple base-config workaround.
This removes the base-config sentinel pointer and keeps the iOS and sandboxed macOS fallback path explicit.
Updates #1338
Change-Id: I836417c8fa775b35d3be9bc80cf6841d30cec222
Signed-off-by: Maisem Ali <maisem@bold.dev>