Introduce common-debug.h

Introduce common-debug.h

This introduces common-debug.h.  This holds the functions debug_printf
and debug_vprintf, two functions that the common code can use to print
debugging messages.  Clients of the common code are expected to
implement debug_vprintf; a debug_vprintf function is written from
scratch for GDB, and gdbserver's existing debug_printf is repurposed
as debug_vprintf.

common/agent.c is changed to use debug_vprintf rather than
defining the macro DEBUG_AGENT depending on GDBSERVER.

nat/i386-dregs.c is changed to use the externally-implemented
debug_printf, rather than defining it itself.

gdb/ChangeLog:

	* common/common-debug.h: New file.
	* common/common-debug.c: Likewise.
	* debug.c: Likewise.
	* Makefile.in (SFILES): Add common/common-debug.c.
	(HFILES_NO_SRCDIR): Add common/common-debug.h.
	(COMMON_OBS): Add common-debug.o and debug.o.
	(common-debug.o): New rule.
	* common/common-defs.h: Include common-debug.h.
	* common/agent.c (debug_agent_printf): New function.
	(DEBUG_AGENT): Redefine.
	* nat/i386-dregs.c (debug_printf): Undefine.

gdb/gdbserver/ChangeLog:

	* Makefile.in (SFILES): Add common/common-debug.c.
	(OBS): Add common-debug.o.
	(common-debug.o): New rule.
	* debug.h (debug_printf): Don't declare.
	* debug.c (debug_printf): Renamed and rewritten as...
	(debug_vprintf): New function.