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

include/sys/zfs_sysfs.h

@@ -23,7 +23,7 @@
  */
 
 #ifndef	_SYS_ZFS_SYSFS_H
-#define	_SYS_ZFS_SYSFS_H
+#define	_SYS_ZFS_SYSFS_H extern __attribute__((visibility("default")))
 
 #ifdef _KERNEL
 
@@ -35,7 +35,7 @@ void zfs_sysfs_fini(void);
 #define	zfs_sysfs_init()
 #define	zfs_sysfs_fini()
 
-boolean_t zfs_mod_supported(const char *, const char *);
+_SYS_ZFS_SYSFS_H boolean_t zfs_mod_supported(const char *, const char *);
 #endif
 
 #define	ZFS_SYSFS_POOL_PROPERTIES	"properties.pool"