add copy of basename
behave like 'cp' if target is a directory

except the case

  megacp . /foo

does not work yet.
use $TMPDIR or /tmp, not the cwd for tempfiles by default
fully behave like 'cp' if target is a directory
add xbacktick
add xtempdir, xtouch

(the 'Shell' module is going to be removed, don't want dependency)
add copies of xmkdir, xchdir, xfork
add test/targetbase
even more fully behave like 'cp' if target is a directory