Bastian Kleineidam: added 'remove_tree()' function. Needed so that

'remove_tree()' can cooperate with 'mkpath()' in the maintenance of
the PATH_CREATED cache: specifically, if a directory is created
with 'mkpath()', later removed with 'remove_tree()', and 'mkpath()'
is again requested to create it, then it would erroneously think
the directory already existed, because it was in the PATH_CREATED
cache.  The patch (slightly tweaked by me) fixes that.