set stream orientations in open_[w]memstream

set stream orientations in open_[w]memstream

fundamentally there is no good reason these functions need to set an
orientation (morally it should be possible to write a wchar_t[] memory
stream using byte functions, or a char[] memory stream using wide
functions), but it's a part of the specification that they do. aside
from being able to inspect the orientation with fwide, failure to set
the orientation in open_wmemstream is observable if the locale changes
between open_wmemstream and the first operation on the stream; this is
because the encoding rule (locale) for the stream is required to be
bound at the time the stream becomes wide-oriented.

for open_wmemstream, call fwide to avoid duplicating the logic for
binding the encoding rule. for open_memstream it suffices just to set
the mode field in the FILE struct.