MFC r260866,260900,261960 (3 revisions; summarized below):
260866: Fix a bad comparison operator and fix dumpdev comment
260900: Dummy commit; show r260866 was in-part by Christoph Mallon
261960: Add zfsboot to bsdinstall(8) scripted interface
MFC: r264349

mdoc and spelling cleanup.
MFC: r264362

Remove stray .Nm macro.