Fix XEmacs compilation warnings and errors in MH-E.

* mh-e.el: Just require mh-loaddefs since loading it in an
eval-and-compile block causes compilation errors in XEmacs.

* mh-acros.el, 	mh-comp.el, mh-e.el, mh-folder.el, mh-letter.el:
* mh-mime.el, mh-search.el, mh-seq.el: Shush XEmacs compiler in
mh-do-in-xemacs block.

* mh-compat.el (mh-window-full-height-p): Add compatibility function
for XEmacs.
* mh-show.el (mh-show-msg): Use it, and avoid compiler warning on
XEmacs.

* mh-letter.el (mh-letter-mode-map, mh-letter-complete)
(mh-complete-word): Remove FIXME comments since these functions are
still needed in other Emacsen. However, they can probably stand to be
generalized like completion-at-point.
(mh-letter-complete-or-space): Remove unused variable.

* mh-compat.el (mh-test-completion): Add compatibility function for
XEmacs.
* mh-alias.el (mh-alias-letter-expand-alias): Use it, and avoid
compiler warning on XEmacs.

* mh-utils.el:
* mh-mime.el: Shush XEmacs compiler in mh-do-in-xemacs block.

* mh-folder.el: Use boundp instead of fboundp when testing
existence of desktop-buffer-mode-handlers.