libzfs: convert to -fvisibility=hidden

Also mark all printf-like funxions in libzfs_impl.h as printf-like
and add --no-show-locs to storeabi, in hopes diffs will make more sense
in future

This removes these symbols from libzfs:

D nfs_only
T SHA256Init
T SHA2Final
T SHA2Init
T SHA2Update
T SHA384Init
T SHA512Init
D share_all_proto
D smb_only
T zfs_is_shared_proto
W zpool_mount_datasets
W zpool_unmount_datasets

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12048