fix uncommon bug in parsing dynare command line options

previously, `nopathchange` and `nopreprocessoroutput` were set even if they were values instead of option names.

`nopathchange` would further remove all options that contained `'nopathchange'`

e.g. `dynare example1.mod savemacro=nopathchange` would erroneously set `nopathchange` to true and would delete the `savemacro` option altogether

In the fix, just check that the match starts in position 1 as, if the argument passed is longer than the matching pattern (e.g. nopathchangee), the preprocessor will stop processing with a usage error