Move buffered stream to new files

The buffered stream code is currently in ui-out.h and ui-out.c, which
seems weird because it seems more like a low-level ui-file idea (for
the most part, it does also affect some ui_out).

This patch moves the declarations to a new header file,
buffered-streams.h and the implementation to buffered-streams.c.  This
seems cleaner to me.

Approved-By: Andrew Burgess <aburgess@redhat.com>