Use /dev/null for stdout/stderr in libzfs_run_process().

Use /dev/null for stdout/stderr in libzfs_run_process().

Simply closing the stdout and/or stderr file descriptors for
the child process can have bad side effects if for example
the child writes to stdout/stderr after open()ing a file.
The open() call might have returned the same file descriptor
one would usually expect for stdout/stderr (1 and 2), thereby
causing mis-directed writes.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #190