Eliminate an #ifdef illumos for zfs_ioc_rename().

Since allow_mounted is a FreeBSD-specific change, default to B_TRUE, then
locally check for the magic bit. Unconditionally check allow_mounted below.
Convert the setting of allow_mounted to an explicit boolean.

MFC after: 1 week
Sponsored by: Spectra Logic
MFSpectraBSD: 672578 (in part) on 2013/07/19