deb: give the build-user setup and resume cleanup their own phases

The profile's build-dependency phase spanned everything between the
apt passes and the build step, and for a kernel incremental build
that hid ~13 minutes of housekeeping: the recursive chown handing
the staged tree to the build user, and the resume_clear quirk
dropping the previous attempt's stamp caches. Both now enter the
profiler (ClearingResumeCaches only when the quirk has paths and a
tree is carried, PreparingBuildUser only for rootless builds), so
the status bar and pkh deb profile show where the time actually
goes — on a large carried tree the chown dominates, which is the
honest answer to a 6m30s DEPS column next to a 19m19s phase.

README.md

@@ -175,10 +175,10 @@ pkh deb list            # the records of this tree: id, outcome,
 pkh deb log [-f] [<id>] # print a record's build log; -f follows a
                         # live build
 pkh deb profile [<id>]  # a record's per-phase timing profile: how
-                        # long the chroot bootstrap, each apt phase,
-                        # patch application, the build and binary
-                        # steps and the artifact retrieval took, with
-                        # their shares of the total
+                        # long every step took (chroot bootstrap,
+                        # apt phases, patch application, build-user
+                        # setup, build and binary steps, artifact
+                        # retrieval), with their shares of the total
 pkh deb --no-ccache     # build without the shared compile cache
 ```