avoid backquote in error messages, some uses in doc strings * libinterp/corefcn/quadcc.cc, libinterp/interpfcn/file-io.cc, libinterp/octave-value/ov-java.cc, scripts/image/colormap.m, scripts/image/rgbplot.m, scripts/miscellaneous/xor.m, scripts/plot/print.m, scripts/special-matrix/gallery.m, scripts/statistics/base/quantile.m, scripts/strings/strsplit.m: Use ' instead of ` in error messages. Don't use sprintf unnecessarily in error messages. Use @code or @samp instead of `' in doc strings.

%!test
%! addpath dir1
%! [d1_r, d1_f1, d1_f2, d1_f3] = func1 (0);
%! addpath dir2
%! [d2_r, d2_f1, d2_f2, d2_f3] = func1 (0);
%! assert (d1_r, 0);
%! assert (d2_r, 1);
%! assert (d1_f1, "dir1/func1");
%! assert (d1_f2, "dir1/func2");
%! assert (d1_f3, "dir1/func3");
%! assert (d2_f1, "dir2/func1");
%! assert (d2_f2, "dir2/func2");
%! assert (d2_f3, "dir2/func3");