Binding `default-*-coding-system' to nil causes XEmacs 21.5.x to signal invalid-state

[Stephen J. Turnbull]

GNU Emacs applications occasionally bind default-process-coding-system
to nil.  For example, uudecode.el from Gnus does this.  TRAMP's ssh
method will trigger this every time.

This causes XEmacs to get upset and signal `invalid-state', because we
test for that being a cons (src/process.c, l. 533).

Similar considerations apply to default-network-coding-system.

Do we really need to be this strict?  If so, what's appropriate here?
(binary . binary)?

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