dprintf-style agent can't explain a trap.

If some event happens to trigger at the same address as a dprintf-style
agent dprintf is installed, GDB will complain, like:

 (gdb) continue
 Continuing.
 May only run agent-printf on the target
 (gdb)

Such dprintfs are completely handled on the target side, so they can't
explain a stop, but GDB is currently putting then on the bpstat chain
anyway, because they currently unconditionally use bkpt_breakpoint_hit
as breakpoint_hit method.

gdb/
2014-06-02  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (dprintf_breakpoint_hit): New function.
	(initialize_breakpoint_ops): Install it as dprintf's
	breakpoint_hit method.