staprun: reinstate -d '*' operation

staprun: reinstate -d '*' operation

PR14245 created a new relay_basedir_fd variable, for use when
staprun/stapio passes staprun a file descriptor for the base directory
of the sysfs systemtap-module directory.  (This is necessary in order
to get around 0700 mount-dir permissions.)  This broke staprun -d '*'
operation, since that variable got set (badly) once within the readdir
loop, and thence made actual removes inoperative.  Now we clear out
that variable so each actual module subdirectory will get a good
cleaning.