use thread_local for ntoa buf

*** 681,691 ****
  #if defined(sparc)
          void            *ul_unwind_ret; /* used only by _ex_clnup_handler() */
  #endif
          tumem_t         ul_tmem;        /* used only by umem */
          uint_t          ul_ptinherit;   /* pthreads sched inherit value */
-         char            ul_ntoabuf[18]; /* thread-specific inet_ntoa buffer */
  } ulwp_t;
  
  #define ul_cursig       ul_cp.s.cursig          /* deferred signal number */
  #define ul_pleasestop   ul_cp.s.pleasestop      /* lwp requested to stop */
  #define ul_curplease    ul_cp.curplease         /* for testing both at once */
--- 681,690 ----