gtkmountoperation: Drop the new line character when splitting a message

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 GNOME/gvfs!82. 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.