Add ui_file::wrap_here

Right now, wrap_here is a global function.  In the long run, we'd like
output streams to be relatively self-contained objects, and having a
global function like this is counter to that goal.  Also, existing
code freely mixes writes to some parameterized stream with calls to
wrap_here -- but wrap_here only really affects gdb_stdout, so this is
also incoherent.

This step is a patch toward making wrap_here more sane.  It adds a
wrap_here method to ui_file and changes ui_out implementations to use
it.

gdb/cli-out.c

@@ -243,7 +243,7 @@ cli_ui_out::do_wrap_hint (int indent)
   if (m_suppress_output)
     return;
 
-  wrap_here (indent);
+  m_streams.back ()->wrap_here (indent);
 }
 
 void