Poll with zero or negative timeout

[Jean-Christian de Rivaz]

Hello,

I tried to use the bug tracking system, but the "Report Issue" page 
never go after the "Select Project" form. So I post this patch here.

The attached patch solve an issue I faced while using the libdbus-glib 
waiting for a D-Bus message or the end of a glib timer at the same time. 
This specific case of use generate a poll call with a zero timeout. On 
platformes with the glibc a zero timeout poll return immetiately even if 
there is no file descriptor event. But on platformes with uClibc a zero 
timeout poll block until a file descriptor event occurs.

I found that the file libc/sysdeps/linux/common/poll.c only take care of 
  positive timeout and pass a null pointer in case of a zero or negative 
timeout, making the zero and negative the same case: blocking.

The patch alway pass a valid structure for the timeout of poll and 
assign acceptable timeout values for the zero and negative case by 
avoiding the math needed for positive timeout value.

This has been tested on a Blackfin BF533 processor.

Have a good day,
--
Jean-Christian de Rivaz
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