Addition of constant to devctl.h

[Joel Sherrill]

Hi

The FACE Technical Standard Edition 3.0 requires that devctl.h define
the constant SOCKCLOSE which may be used with posix_devctl().

I don't think any other target supports posix_devctl() and AFAIIK no
OS supprting devctl.h which isn't also trying to follow the FACE
Technical Standard. For newlib, that set is just RTEMS.

My best solution so far is to add it at the bottom of  the rtems
specific sys/sockio.h and conditionalize the include of <sys/sockio.h>
in devctl.h on __rtems__.

Any thoughts?

--joel
.