Execute a program on the system and return a handle to the process. Otherwise, takes the same arguments as `os/execute`. Does not wait for the process. For each of the :in, :out, and :err keys of the `env` argument, one can also pass in the keyword `:pipe` to get streams for standard IO of the subprocess that can be read from and written to. The returned value `proc` has the fields :in, :out, :err, and the additional field :pid on unix-like platforms. `(os/proc-wait proc)` must be called to rejoin the subpr