Back when we had RAC this was a vaguely meaningful thing.  Since then
it's been a glorified (and confusing) wrapper around xf86BlockSIGIO.

Note that the APM and VT switch code are unusual relative to other code
that cares about SIGIO state.  Most callers push a SIGIO disable to
create a critical section for the duration of the caller's stack frame,
but those two effectively disable SIGIO after their return and re-enable
on their next entry.

Reviewed-by: Tiago Vignatti <tigo.vignatti@nokia.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>