(c2rust-analyze) Support custom deallocators/linear Box types

[kkysen]

For custom allocator/deallocator functions other than {m,c,re}alloc and free, such as:

• getaddrinfo and freeaddrinfo
• open, etc. and close
• glob and globfree
• mmap and munmap

it would be nice to also support them like we do for the primary allocators/deallocator. As @spernsteiner said,

maybe we could handle this with an additional permission or flag that translates into a custom "linear (non-affine) box" type that must be explicitly freed with a special function (you can't just let it go out of scope).

That said, even if we don't track them correctly, memory leaks are safe, and we'll be doing the same memory management as was in C. It'd be similar to any other resource as well, like open, etc. and close.

• See also (c2rust-analyze) Add almost all libc KnownFns used in lighttpd_rust_amalgamated #998 (comment).