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.