Remap existing EAI_SYSTEM to avoid relying on global errno variable

In order to not rely on errno, remap the existing error code pair of EAI_SYSTEM
and errno to other EAI_* error codes. Unused errno is removed as well.

Test: built, flashed, booted
      system/netd/tests/runtests.sh pass

Change-Id: I8a4450df3c27a12db790756ba545787bd0570402