dtrace(1): add -d flag to dump D script post-dt_sugar

By specifying the -d flag, libdtrace will dump the D script after it has
applied syntactical sugar transformations (e.g if/else). This is useful
for both understanding what dt_sugar does, as well as debugging it.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38732