2668. [func] Several improvements to dnssec-* tools, including:

2668.	[func]		Several improvements to dnssec-* tools, including:
			- dnssec-keygen and dnssec-settime can now set key
			  metadata fields 0 (to unset a value, use "none")
			- dnssec-revoke sets the revocation date in
			  addition to the revoke bit
			- dnssec-settime can now print individual metadata
			  fields instead of always printing all of them,
			  and can print them in unix epoch time format for
			  use by scripts
			[RT #19942]