Changes

build: move netns/tun conditionals to configure.ac
Move Linux-/embedded-specific conditional compilation out of the
sources and into configure.ac.

- Define USE_NETNS based on host_os and disable it for --enable-embedded.
- Detect struct in6_addr.s6_addr32 via AC_CHECK_MEMBER and use
  HAVE_IN6_ADDR_S6_ADDR32 instead of __linux__.
- Do not build tun.c for embedded targets (previously it was compiled
  but effectively empty due to #if (!EMBEDDED)).

No functional changes intended, this is a preparatory refactor to keep
follow-up patches focused and easier to review.

Change-Id: I2322eb2936bea35596f1fd6b6a713ea5f997b1ea