gtkmountoperation: Drop the new line character when splitting a message

When asking for a password, the message string is split on primary and secondary if it contains a newline character. However, the newline character is currently part of both strings, which creates weird spacing between the GtkLabels. I suppose this is bug, which was not visible as in most cases (if not all) the message string hasn't contained the new line characters so far. But we are going to change that now, see gvfs!82 (merged). Let's drop the new line character similarly as it is done when asking for a question, or showing processes in order to fix the weird spacing.