gdb: add debug_prefixed_vprintf

gdb: add debug_prefixed_vprintf

To help ensure that all debug statements have the same format, introduce
the debug_prefixed_vprintf helper.  Implement linux_nat_debug_printf_1
and infrun_debug_printf_1 with it.

I would eventually like to style the module and function name with some
color, to help them stick out, but I don't really know how to do that
yet, it can always be done later.

gdb/ChangeLog:

	* debug.h: New file.
	* debug.c (debug_prefixed_vprintf): New function.
	* infrun.c (infrun_debug_printf_1): Use debug_prefixed_vprintf.
	* linux-nat.c (linux_nat_debug_printf_1): Likewise.

Change-Id: Iccc290a2dc6b5fffcbe1c2866ed8d804ad380764