lib{efi,avl,share,tpool,zfs_core,zfsbootenv,zutil}: -fvisibility=hidden

No symbols affected in libavl
No symbols affected by libtpool, but pre-ANSI declarations got purged
No symbols affected by libzfs_core
No symbols affected by libzfs_bootenv

libefi got cleaned, gained efi_debug documentation in efi_partition.h,
and removes one undocumented and unused symbol from libzfs_core:
  D default_vtoc_map

libnvpair saw removal of these symbols:
  D nv_alloc_nosleep_def
  D nv_alloc_sleep
  D nv_alloc_sleep_def
  D nv_fixed_ops_def
  D nvlist_hashtable_init_size
  D nvpair_max_recursion

libshare saw removal of these symbols from libzfs:
  T libshare_nfs_init
  T libshare_smb_init
  T register_fstype
  B smb_shares

libzutil saw removal of these internal symbols from libzfs_core:
  T label_paths
  T slice_cache_compare
  T zpool_find_import_blkid
  T zpool_open_func
  T zutil_alloc
  T zutil_strdup

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12191

include/sys/avl_impl.h

@@ -25,8 +25,7 @@
  */
 
 #ifndef	_AVL_IMPL_H
-#define	_AVL_IMPL_H
-
+#define	_AVL_IMPL_H extern __attribute__((visibility("default")))
 
 
 /*
@@ -155,7 +154,7 @@ struct avl_tree {
 /*
  * This will only by used via AVL_NEXT() or AVL_PREV()
  */
-extern void *avl_walk(struct avl_tree *, void *, int);
+_AVL_IMPL_H void *avl_walk(struct avl_tree *, void *, int);
 
 #ifdef	__cplusplus
 }