Import tsort(1) from FreeBSD r227189
tsort: Add -D to show depth of node
Use depth of dependency chain for priority, not direct deps
compute_deps: Speedup 9% by not creating unneeded rdeps symlink anymore
Rename port deps depth file
compute_deps: Speedup 13% by not extremely redundantly calling mkdir(1) for rdeps
Move dep calc code into compute_deps and rename compute_deps to compute_deps_port