minor cleanup up libshare
The attached patch is a minor cleanup of libshare. It fixes the following things:
1. sa_handle_impl_t and sa_handle_t are no longer separate types, although sa_handle_t is an opaque type unless libshare_impl.h is included.
2. sa_is_share, sa_is_resource, sa_group_is_zfs, sa_path_is_zfs, sa_is_persistent, sa_is_security and sa_zfs_is_shared now return boolean_t.
3. ZFS_MAXPROPLEN is used in place of MAXPATHLEN for ZFS property buffers.
4. sa_get_zfs_info has been removed because it was not used anywhere. It was also never exported.
5. libzfs_init() is now only called once, inside sa_zfs_init.