Clarified the role of MDOC_HALT in libmdoc functions by having accessor

functions assert() if they're called after MDOC_HALT is set.

This makes more sense than returning 0 because this return value is used
for parse errors, not programme-flow errors, and it's inconsistent to
use the same value for both.  Plus, prior to this, I'd return 0 without
printing an error message, which would cause failure to go unreported to
the operator.

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