libzfs: get rid of libzfs_handle::libzfs_mnttab

libzfs: get rid of libzfs_handle::libzfs_mnttab

All users did a freopen() on it. Even some non-users did!
This is point-less ‒ just open the mtab when needed

If I understand Solaris' getextmntent(3C) correctly, the non-user
freopen()s are very likely an odd, twisted vestigial tail of that ‒
but it's got a completely different calling convention and caching
semantics than any platform we support

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11868