x11-plugins/enigmail: install extension so that it will be dynamically loaded

x11-plugins/enigmail: install extension so that it will be dynamically loaded

Instead of installing files in a generic location that then requires
compile-time support from thunderbird or seamonkey to integrate, starting with
v1.9.8.3-r1 enigmail now installs in a path that relevant mozilla products will
load from on startup.  This means that USE=crypt on seamonkey and thunderbird
will be going away, as the installation of enigmail now only requires adding
this package to @world.

Package-Manager: Portage-2.3.13, Repoman-2.3.3