build: detect netns support via configure-time capability checks
The netns API was previously guarded by platform-specific preprocessor conditions (e.g. !EMBEDDED and defined(__linux__)), which caused the netns headers and implementation to be entirely excluded from non-Linux builds such as Emscripten. This in turn led to build failures in code depending on the netns API, as the symbols and declarations were not available at all.
Replace the hard-coded platform checks with explicit configure-time capability detection. The build now checks for the concrete requirements needed by netns and netdev support, including the availability of setns(), unshare(), mount(), the CLONE_NEWNET macro and required headers.
Based on the result of these checks, USE_NETNS is defined and netns support is either enabled or disabled in a controlled manner.
Netns and netdev support remains disabled for embedded builds.
No functional changes intended for platforms where netns is available.
