fix for PR gdb/15995

This patch fixes PR gdb/15995.

The bug here is that gdb's printf command does not flush the output
stream.  This makes a printf that is not newline-terminated interleave
incorrectly with other forms of output, such as that generated via a
call to an external program using "shell".

I note that the "output" command already does this flushing.

The fix is to call gdb_flush in printf_command.

Built and regtested on x86-64 Fedora 18.
New test case included.

	PR gdb/15995:
	* printcmd.c (printcmd): Call gdb_flush.

	* gdb.base/printcmds.exp (test_printf): Test printf flushing.