multiprocess: Add libmultiprocess git subtree

This adds the libmultiprocess library and code generator as a subtree in src/ipc/libmultiprocess and allows it to be built with the cmake -DENABLE_IPC option, which is disabled by default.

This PR does not entirely remove the depends system libmultiprocess package because the package is useful when cross compiling. (A cross-compiling cmake build cannot easily build and run a native code generation tool.) However, it does update the depends package to build from the new git subtree, instead of being downloaded separately from github, so the same sources are used to build both the runtime library and the code generator.

This PR includes the following manual changes (not created automatically with git subtree add) which just update the build system and documentation:

• 1746618e081d depends: Update libmultiprocess library before converting to subtree
• 8eccd5322da1 scripted-diff: s/WITH_MULTIPROCESS/ENABLE_IPC/ in cmake
• 8b68709ce378 cmake: Support building with libmultiprocess subtree
• 5c2c09cdfa61 cmake: Fix ctest mptest "Unable to find executable" errors
• 6b350f593fef cmake: Fix warnings from boost headers
• 45bb7fed27d6 cmake: Fix clang-tidy "no input files" errors
• 96d3b2a0bb0c cmake: Fix fuzzer "multiple definition of `main'" errors
• 20a804942afd doc: Update documentation to explain libmultiprocess subtree
• d8cbd577d061 lint: Add exclusions for libmultiprocess subtree
• 15a06000b086 depends, moveonly: split up int_get_build_id function
• ca1c8ef7d833 depends: Switch libmultiprocess packages to use local git subtree
• b2108240dec5 depends: remove non-native libmultiprocess build

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Previous minisketch subtree PR #23114 may be useful for comparison

Instructions for subtree verification can be found:

• https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#subtrees
• https://github.com/bitcoin/bitcoin/tree/master/test/lint#git-subtree-checksh

TL&DR:

git remote add --fetch libmultiprocess https://github.com/chaincodelabs/libmultiprocess.git
test/lint/git-subtree-check.sh -r src/ipc/libmultiprocess

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This PR is part of the process separation project.