build: keep netns API public and gate features with HAVE_*
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.
Always build netns.c and keep the public declarations available for non-embedded builds. Move platform/feature conditionals into the implementation: guard the netns code paths with HAVE_SETNS/HAVE_UNSHARE/ HAVE_MOUNT and HAVE_CLONE_NEWNET, and return -ENOTSUP when the required functionality is not available.
Add configure-time checks for the required headers/functions and for the CLONE_NEWNET declaration, defining the corresponding HAVE_* macros.
No functional changes intended for platforms where netns is available.
