[lib] Add ZSTD_c_deterministicRefPrefix

[lib] Add ZSTD_c_deterministicRefPrefix

This flag forces zstd to always load the prefix in ext-dict mode, even
if it happens to be contiguous, to force determinism. It also applies to
dictionaries that are re-processed.

A determinism test case is also added, which fails without
`ZSTD_c_deterministicRefPrefix` and passes with it set.

Question: Should this be the default behavior? It isn't in this PR.