Reduce use of unfiltered output in Darwin code

Reduce use of unfiltered output in Darwin code

The Darwin code uses unfiltered output liberally.  This patch changes
this code to send some output to gdb_stdlog (in some cases via the use
of debug_prefixed_printf_cond_nofunc), or to gdb_stderr, or to simply
switch to filtered output.

Note that I didn't switch inferior_debug to use
debug_prefixed_printf_cond_nofunc, because that would affect the
output by removing the information about the inferior.  I wasn't sure
if this was important or not, so I left it in.

v2 of this patch uses warning rather than prints to gdb_stderr, and
removes some trailing whitespace.

I can't compile this patch, so it's "best effort".