Make out and error streams be per UI

Make out and error streams be per UI

stderr_fileopen () references stderr directly, which doesn't work when
we have a separate UI with its own stderr-like stream.  So this also
adds a "errstream" to "struct ui", and plumbs stderr_fileopen to take
a stream parameter.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* event-top.c (gdb_setup_readline): Pass the UI's outstream and
	errstream to stdout_fileopen and stderr_fileopen.
	* exceptions.c: Include top.h.
	(print_flush): Open the current UI's outstream file descriptor,
	instead of hardcoding file descriptor 1.
	* main.c (captured_main): Save the main UI's out and error
	streams.  Adjust stderr_fileopen call.
	* top.h (struct ui) <outstream, errstream>: New fields.
	* ui-file.c (stderr_fileopen): Add stream parameter.  Use it
	instead of stderr.
	* ui-file.h (stderr_fileopen): Add stream parameter and update
	comment.