MFC r304920:

In ncurses baudrate definitions, avoid warnings about implicit
conversions from int to short changing the values. This applies to
B38400 and higher, since their values do not fit into a short.

However, since the wrapped values are still unique, and they only serve
as keys, there is no problem in adding a cast to silence the warnings.
This also avoids changing the ABI, which would happen if we changed
NCURSES_OSPEED to int.

Discussed with: Thomas Dickey