inappropriate encoding of TUTORIAL.bg

[Dave Love]

etc/TUTORIAL.bg is encoded as koi8-r, which isn't appropriate.  The
GNU Bulgarian locale uses the windows-1251 charset (which it calls
`cp1251') and the Bulgarian language environment follows that.

If this is because windows-1251 isn't available by default, the
solution is to make it so.  I think at least the coding systems in
code-pages.el which are necessary to support GNU locales should be
preloaded (or autoloaded).

Related to this, I also think the codepage.el coding systems should be
confined to DOS and not be sucked in by special treatment in
mule-diag.el, for instance.  I think I have a version of the relevant
files suitably expurgated so that code-pages.el doesn't have to take
its own measures to avoid trouble..