This is similar to `rosenpass exchange`/`rosenpass exchange-config`.
It's however slightly different to the configuration file models the `rp
exchange` command line.
- Policy is now set in main.rs, not cli.rs.
- Feature is called experiment_memfd_secret, not enable_memfd_alloc
This also fixes the last remaining warnings.
Implements:
- An additional allocator to use memfd_secret(2) and guard pages using mmap(2), implemented in quininer/memsec#16
- An allocator that abstracts away underlying allocators, and uses specified allocator set by rosenpass_secret_memory::policy functions (or a function that sets rosenpass_secret_memory::alloc::ALLOC_INIT
- Updates to tests- integration, fuzz, bench: some tests use procspawn to spawn multiple processes with different allocator policies
