stand: Make ioctl declaration consistent

It typically had two args with an optional third from the userland
declaration in sys/ioccom.h. However, the funciton definition used a
non-optional char * argument. This mismatch is UB behavior (but worked
due to the calling convetions of all our machines).

Instead, add a declaration for ioctl to stand.h, make the third arg
'void *' which is a better match to the ... declaration before. This
prevents the convert int * -> char * errors as well. Make the ioctl
user-space declaration truly user-space specific (omit it in the
stand-alone build).

No functional change intended.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37680