pioctl.c: restore required result variable

pioctl.c: restore required result variable

Commit b9fb9c62a6779aa997259ddf2a83a90b08e04d5f refactored lpioctl()
so that LINUX would have its own implementation. This also simplified
the other lpioctl() implementations by removing superfluous variable
'rval'.

Unfortunately, 'rval' was actually required for both DARWIN and SUN511.
On both of these platforms, the address of 'errcode' is passed
to the respective ioctl_*() routine so its value may be passed back
to lpioctl().   Therefore, 'errcode' must not also be used for the
return value from these functions;  doing so results in the return
value from the function overwriting the intended value of 'errcode' upon
return to lpioctl().

In the case of Solaris 11, ioctl_sun_afs_syscall() always returns zero
(as long as the ioctl device 'dev/afs' opened successfully).
So 'errcode' was always being set to zero, even if the pioctl had
actually failed.  For example, without this fix, 'fs listcells'
loops forever on Solaris 11, listing an infinite number of "cells",
because it will never "see" the EDOM that informs it of the last defined
cell.

Partially revert b9fb9c62a6779aa997259ddf2a83a90b08e04d5f by restoring
the 'rval' variable and logic for DARWIN and SUN511.

Change-Id: I4407af29d54813689cf8ccf2517bb2df4dd8eb25
Reviewed-on: http://gerrit.openafs.org/11734
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>