Major TLS cleanups.

Major TLS cleanups.

* The RTLD no longer attempts to set up an initial TLS.  The job has been
  moved to libc.  The RTLD does not use TLS itself.  The
  allocate_initial_tls() function has been removed.

* The _init_tls() call made by CRT1 is no mandatory.

* The RTLD no longer calls the library _init() functions early.  Instead it
  sets up a callback that allows CRT1 to call the library _init() functions
  after it has called _init_tls().

* LIBC now has a _libc_init_tls() function which is weakly bound as
  _init_tls().  This function initializes the TLS for the program (generally
  by calling the _rtld_allocate_tls() and then doing any required
  post-allocation initialization).  In the absence of a threading library
  this function is basically it.

  Threading libraries may replace _init_tls() with their own version.  This
  does not yet occur as of this commit.  The threading library would then
  almost certainly call _libc_init_tls() and then make further modifications
  to the TCB as necessary (e.g. populate the tcb_pthread field).

* RTLD's allocate_tls() no longer does anything fancy.  The 'old_tcb' argument
  and concept has been removed.

* We have a new tcb_errno_p field in the TCB, but as of this commit it
  is not yet being used.

Reviewed-by: Joerg Sonnenberger <joerg@britannica.bec.de>,
		David Xu <davidxu@crater.dragonflybsd.org>