Use setmntent() OR fopen()

For the same reasons it's used in libzfs_init(), this was just
overlooked because zinject gets minimal use.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1498
No point in rewind() mtab in zfs_unshare_proto(). We're not really
reading the file, but instead use libzfs_mnttab_find() which does
the nessesary freopen() for us.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1498
Don't specifically open /etc/mtab - it is done in libzfs_init()
a few lines further down and we can share the open file handle.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1498