general code cleanup

- strings like  "\"" or "\("R can be expressed more easily
  as backquoted strings (namely as `"` and `\(`), as no
  backslash substitution is performed on backquoted strings.

- A $ suffix enables variable substitution on a string:
  e.g.,          "mv $filename.eps.tmp $filename.eps"$
  corresponds to "mv "+filename+".eps.tmp "+filename+".eps"
  and "%.${n}f"$
   to "%."+string(n)+"f";

- `if(not x)' can be simplified to `ifnot(x)'

- The `get1match' and `string_match_perl' functions
  are now deprecated in favour of S-Lang's `string_matches'.

- Some of the FITS file functions indicate in their help
  that there are more direct ways using cfitsio module functions.
  `fits_modify_header' has been deprecated in favour of fits_update_key.