make dprintf.exp pass in target async mode

When target-async is enabled, dprintf.exp fails:

 Running ../../../src/gdb/testsuite/gdb.base/dprintf.exp ...
 FAIL: gdb.base/dprintf.exp: 1st dprintf, call
 FAIL: gdb.base/dprintf.exp: 2nd dprintf, call
 FAIL: gdb.base/dprintf.exp: Set dprintf function
 FAIL: gdb.base/dprintf.exp: 1st dprintf, fprintf
 FAIL: gdb.base/dprintf.exp: 2nd dprintf, fprintf

 Breakpoint 2, main (argc=1, argv=0x7fffffffd3f8) at ../../../src/gdb/testsuite/gdb.base/dprintf.c:33
 33        int loc = 1234;
 (gdb) continue
 Continuing.
 kickoff 1234
 also to stderr 1234
 At foo entry
 (gdb) FAIL: gdb.base/dprintf.exp: 1st dprintf, call

The problem is that GDB gave the prompt back to the user too early.

This happens when calling functions while handling an event that
doesn't cause a user visible stop.  dprintf with "set dprintf-style
gdb" is one such case.  This patch adds a test case that has a
breakpoint with a condition that calls a function that returns false,
so that regression testing isn't dependent on the implementation of
dprintf.

The problem happens because run_inferior_call causes GDB to forget
that it is running in sync_execution mode, so any event that runs an
inferior call causes fetch_inferior_event to display the prompt, even
if the event should not result in a user visible stop (that is, gdb
resumes the inferior and waits for the next event).

This patch fixes the issue by noticing when GDB was in sync_execution
mode in run_inferior_call, and taking care to restore this state
afterward.

gdb/
2014-03-20  Tom Tromey  <tromey@redhat.com>

	PR cli/15718
	* infcall.c: Include event-top.h.
	(run_inferior_call): Call async_disable_stdin if needed.

gdb/testsuite/
2014-03-20  Tom Tromey  <tromey@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR cli/15718
	* gdb.base/condbreak-call-false.c: New file.
	* gdb.base/condbreak-call-false.exp: New file.