XXX
Introduce and use debug_printf and debug_vprintf

This introduces debug_printf and debug_vprintf, a function that
clients of "common" are expected to implement.  gdbserver's
existing debug_printf is repurposed as debug_vprintf, and a new
wrapper is written.

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.