Reuse loclists when possible, to allow :lolder and :lnewer.

This is not completely safe: if a loclist is created by something
other than syntastic before the error window is displayed, syntastic
will obliterate it.  There is currently no way to tell if a
loclist was created by syntastic or not.  Undocumented variable
g:syntastic_reuse_loc_lists can be used to disable reuse of loclists.
Bug fix: allow loclist cleanup.
Merge branch 'loclist_count'