remove directory entries before directories

[sertonix]

When using samu -t clean samu might try to remove directories before removing their contents. It would be nice if samu would try to remove directory entries first before trying to remove a directory.

rule mkdir
  command = mkdir -p $out $out/baz

build foo | foo/baz: mkdir

$ samu
[1/1] mkdir -p foo foo/baz
$ samu -t clean
samu: remove foo: Directory not empty
remove foo/baz

When dyndep support (#48) will be added this is also required to clean up an extracted tar file with nested directories since it is not possible to create new targets in dyndeps.