Fix #6154 by introducing new EAGAIN/EWOULDBLOCK/EINPROGRESS
subclasses that include WaitReadable or WaitWritable rather than
extending them into the exception object each time.

* error.c: Capture EGAIN, EWOULDBLOCK, EINPROGRESS exceptions and
  export them for use in WaitReadable/Writable exceptions.
* io.c: Create versions of EAGAIN, EWOULDBLOCK, EINPROGRESS that
  include WaitReadable and WaitWritable. Add rb_readwrite_sys_fail
  for nonblocking failures using those exceptions. Use that
  function in io_getpartial and io_write_nonblock instead of
  rb_mod_sys_fail
* ext/openssl/ossl_ssl.c: Add new SSLError subclasses that include
  WaitReadable and WaitWritable. Use those classes for
  write_would_block and read_would_block instead of rb_mod_sys_fail.
* ext/socket/ancdata.c: Use rb_readwrite_sys_fail instead of
  rb_mod_sys_fail in bsock_sendmsg_internal and
  bsock_recvmsg_internal.
* ext/socket/init.c: Use rb_readwrite_sys_fail instead of
  rb_mod_sys_fail in rsock_s_recvfrom_nonblock and
  rsock_s_connect_nonblock.
* ext/socket/socket.c: Use rb_readwrite_sys_fail instead of
  rb_mod_sys_fail in sock_connect_nonblock.
* include/ruby/ruby.h: Export rb_readwrite_sys_fail for use instead
  of rb_mod_sys_fail. Introduce new constants RB_IO_WAIT_READABLE and
  RB_IO_WAIT_WRITABLE for first arg to rb_readwrite_sys_fail.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e