restor/dumpdir tempfiles

[utzoo!henry]

Both dumpdir and the x subcommand of restor create temporary files
with names of the form "rstXXXXXX" in the *current directory*.
Moreover, they are very careless about cleaning up after themselves,
which can lead to annoying junk files left around.  It is the work of
a moment to change them to put these files in /tmp instead.  This may
reduce their usability somehow under conditions of extreme system damage,
but I can't visualize the circumstances.