Extract patch pattern getting into a function

There might be further pattern getters.
Rename the original function to s:SetBufferSpecificPatternPrefix() which
indicates that it does not set the full pattern.
Turn the highlight off initially for selected filetypes

It is now possible to specify filetypes for which no trailing space will
be highlighted initially.  An example for the new variable:

    let g:bad_whitespace_off_filetypes = ['text', 'markdown']
Make the show/hide toggle persistent by clearing autocommands

Clear autocommands when hiding whitespace.