Fixed tags file corruption on Windows!

 * Fixed tags file corruption on Windows: It turns out that the function
   readfile() accepts CR+NL line endings and just strips CR characters,
   however the writefile() function doesn't write CR+NL but just NL.
   Manually adding the CR characters to the end of each line before
   writing the tags file solves the corruption bug.

 * The headers and entries in tags files are now properly separated from
   each other while rewriting the tags file to filter expired entries.

 * Don't bother trying to filter expired tags when the tags file doesn't
   reference the current filename at all (improves performance a bit).

 * Fixed a buggy easytags#file_has_tags() call in easytags#autoload()
   that made the plug-in always scan each edited file at least once.

 * Changed folds to provide basic outline of function categories.

 * Restructured definitions for default configuration of dynamic syntax
   highlighting support so definitions don't all have to be indented.

Sorry about the large diff, this version took a few days of
experimentation to build which means lots of little changes.