ZFS: add bookmark renaming

The feature is implemented as an extension of the existing
ZFS_IOC_RENAME ioctl. Both the userland and the DSL interfaces support
renaming only a single bookmark at a time. As of now, there is no ZCP
interface to the new functionality. I am going to add it once the DSL
interface passes a test of time.

This change picks up support for zfs_ioc_namecheck_t::ENTITY_NAME that
was added to ZoL as part of Redacted Send/Receive feature by Paul
Dagnelie <pcd@delphix.com>. This is needed to allow a bookmark name in
zc_name.

Discussed with: mahrens
Reviewed by: bcr (man page)
Sponsored by: CyberSecure
Differential Revision: https://reviews.freebsd.org/D21795