Unify vprintf functions

Now that filtered and unfiltered output can be treated identically, we
can unify the vprintf family of functions: vprintf_filtered,
vprintf_unfiltered, vfprintf_filtered and vfprintf_unfiltered.  (For
the gdb_stdout variants, recall that only printf_unfiltered gets truly
unfiltered output at this point.)  This removes one such function and
renames the remaining two to "gdb_vprintf".  All callers are updated.
Much of this patch was written by script.