(back-port to 4.0)

Socket timeouts in client library were used only on Windows.
Additionally, in 4.0 write operations erroneously set read timeout.

The solution is to use socket timeouts in client library on all
systems were they are supported, and to differentiate between read
and write timeouts.

No test case is provided because it is impossible to simulate network
failure in current test suite.


include/violite.h:
  Add argument to vio_timeout() to determine which timeout should be set:
  for read (false) or for write (true).
libmysqld/lib_vio.c:
  Add argument to vio_timeout() to determine which timeout should be set:
  for read (false) or for write (true).
sql/net_serv.cc:
  Add argument to vio_timeout() to determine which timeout should be set:
  for read (false) or for write (true).
vio/viosocket.c:
  Add argument to vio_timeout() to determine which timeout should be set:
  for read (false) or for write (true).
  Implement socket timeouts on POSIX systems.