Expand DDB's set of printable soft dependency data structures. The
set of known soft dependency data structures now includes: sd_worklist,
sd_inodedep, sd_allocdirect, sd_allocindir, and sd_mkdir. DDB can
also print lists of sd_allinodedeps, sd_mkdir_list, and sd_workhead.
The sd_workhead script is useful for listing all the dependencies
associated with a buffer, e.g. bp->b_dep.

Prefix the soft dependency show names with sd_ so that they sort
together when listed by DDB's "show help" and to distinguish them
from other data structures printable by DDB.

Sponsored by: Netflix