Silence some gnus compilation warnings

Silence some gnus compilation warnings

* gnus/mm-decode.el: No need to load term when compiling.
(term-mode, term-char-mode): Declare.

* gnus/mm-util.el: No need to load jka-compr when compiling.
(jka-compr-acceptable-retval-list, jka-compr-make-temp-name): Declare.

* gnus/nnmaildir.el: Require is automatically eval-and-compile.
(nnmail): Require at run-time too.

* gnus/registry.el (registry-size): Move definition before use.