Add an option to enable/disable DTrace without disabling ZFS. New
architectures such as CHERI may support ZFS before they support DTrace
and the old model of WITHOUT_CDDL disabling both wasn't helpful.
For compatiblity, the CDDL option remains and WITHOUT_CDDL implies
WITHOUT_DTRACE. WITHOUT_DTRACE also implies WITHOUT_CTF.
As part of this change, largely convert cddl/*/Makefile to using the
more compact SUBDIR.${MK_<FOO>}+= form rather than using intermediate
variables.
Reviewed by: markj
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D35718
3bf6636512