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tcl function: source readenv.tcl -- read lowercase env to tcl
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keyboard equivalent: none

Read lower case shell environment variables (set
in sh/bash by "export", or in csh/tcsh by
"setenv") into the tcl interpreter to
override/create default tcl script parms.

Any environment var starting with a lower case
letter will be global to all scripts:

  sh/bash:   export parm=value
  csh/tcsh:  setenv parm value

To restrict a parameter (e.g., a rendering
parameter) to one scandir, prefix it with the
appropriate $rgbname, e.g., "curv" (which
saveviews.tcl will expand to something like
"/full/path/to/curv-rh-pial-lat.tiff").

  sh/bash:   export $rgbname.parm=value
  csh/tcsh:  setenv $rgbname.parm value

