Linux 3.4 compat, __clear_close_on_exec replaces FD_CLR

torvalds/linux@1dce27c5aa6770e9d195f2bb7db1db3d4dde5591 introduced
__clear_close_on_exec() as a replacement for FD_CLR. Further commits
appear to have removed FD_CLR from the Linux source tree. This
causes the following failure:

error: implicit declaration of function '__FD_CLR'
[-Werror=implicit-function-declaration]

To correct this we update the code to use the current
clear_close_on_exec() interface for readability. Then we introduce
an autotools check to determine if clear_close_on_exec() is available.
If it isn't then we define some compatibility logic which used the older
FD_CLR() interface.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #124