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".