fix up r354333, make zfsproc visible to dtrace, rename to system_proc

I overlooked the fact that zfsproc is required by dtrace modules that
use illumos compatible taskq KPI. So, move the symbol definition to
the opensolaris module that provides compatibility support for both ZFS
and DTrace. Also, rename zfsproc to system_proc to reflect that it is
not specific to ZFS.

Reported by: ae
MFC after: 5 weeks
X-MFC with: ae