partial backport of guido's 1.188.
  Add sendall() method, which loops until all data is written or an
  error occurs, and doesn't return a count.  (This is my second patch
  from SF patch #474307, with small change to the docstring for send().)

the 'partial' is because 1.188 also added a couple of Py_*_ALLOW_THREADS
wrappers around SSL_read and SSL_write, and I want to check those separately.

This is adding a new method to the socket object, which would normally
be a bad thing to do in a bugfix release - however, in this case, it
allows fixes for a nasty problem that would otherwise have a filthy
filthy fix to Get It Right. Still to-do is to patch the std library
modules to use sendall() where appropriate, rather than send().