Add rcParams support for markers' fillstyle prop

This registers the `markers.fillstyle` option with the `rcParams` dict to allow configuring the default fillstyle of plot markers via rc files.

I noticed in the diff that the commit also removes trailing whitespaces in a few places. It's stated in the coding guidelines that trailing whitespace should be avoided. However, if this change isn't desired after all or shouldn't be mixed with actual code changes, I'll split the changes into two separate commits.