More `defcustom` type byte-compiler warnings

[Mattias Engdegård]

The byte-compiler now warns about more `defcustom` type mistakes.
As always, please take care of warnings in code that you know about.
The most common error seems to be misplaced keywords. For example,

  (const vanilla :tag "Vanilla flavour")

should be

  (const :tag "Vanilla flavour" vanilla)

because the keyword arguments must come first, after the type symbol.

If anything is unclear or could be improved, do tell.