io: make max unbounded io worker configurable

Since Linux 5.15 io_uring can limit the number of iow threads created using IORING_REGISTER_IOWQ_MAX_WORKERS.

Expose this via the meson variable io_unbounded_iow_max and the environment variable EMPER_IO_UNBOUNDED_IOW_MAX.

See for an detailed explanation: https://blog.cloudflare.com/missing-manuals-io_uring-worker-pool