mail/exmh2: Chase toolkits/tk87 8.7a5

tk 8.7a5 introduced uniform mouse wheel event handling. Unfortunately this
is incompatible with tcltk software written using Button-4 and Button-5
events. This patch replaces the legacy bindings with the new MouseWheel
binding, using the new tk::MouseWheel helper to move the scrollbar.
This is documented at https://core.tcl-lang.org/tips/doc/trunk/tip/474.md.

The old bindings are left should anyone wish to use exmh2 with the older
tk86, also requiring a user to modify Makefile to alter the dependency.