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

config/Abigail.am

@@ -25,5 +25,5 @@ checkabi:
 storeabi:
 	cd .libs ; \
 	for lib in $(lib_LTLIBRARIES) ; do \
-		abidw $${lib%.la}.so > ../$${lib%.la}.abi ; \
+		abidw --no-show-locs $${lib%.la}.so > ../$${lib%.la}.abi ; \
 	done