libexpr-c: fix changing eval settings having no effect

[numinit]

Motivation

Using the Rust bindings (which call into the C bindings) to set eval-related settings was not working.

Since EvalSettings were defined in libcmd, we need to move them to libexpr if libexpr-c wants to use them. Since libcmd enforces a dependency on libflake, we also need to modify config-global to absorb duplicate registrations and support callbacks for customizing behavior that we only need in libcmd.

Note that we add back some of the global setting variables with this, but this does fix the C API.

Context

Implementation strategy, since this was slightly nontrivial:

• Move registration for eval (and fetcher settings, which had the same issue but was easier to solve) to libexpr and libfetchers.
• Absorb duplicate config registrations in config-global.cc.
• Add a new GlobalConfig registration constructor allowing the caller to pass a callback; use this to break the hard libflake dependency.
• Use pointers to the global eval and fetcher config in libexpr-c instead of creating new instances that are not bound to Nix's settings.

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[numinit]

CC @RossComputerGuy (who I bugged about this issue while tracking it down).

[roberth]

@Ericson2314 as Chief of Settings Cleanup, wdyt?
If I'm not mistaken, this PR pursues the same problem as

• Fix path coercion by calling eval_state_builder_load() nixops4/nix-bindings-rust#35