Use Boost.Process instead of pid_t (#6510)

* Use Boost.Process instead of pid_t

This will let us handle child processes (mostly) uniformly across platforms.
TODO: There is no SIGTERM on Windows; achieving something equivalent is fairly involved.

bazel/ray_deps_setup.bzl

@@ -123,6 +123,8 @@ def ray_deps_setup():
             # Backport Clang-Cl patch on Boost 1.69 to Boost <= 1.68:
             #   https://lists.boost.org/Archives/boost/2018/09/243420.php
             "//thirdparty/patches:boost-type_traits-trivial_move.patch",
+            # Partially backport waitpid() patch on Boost 1.72 to Boost <= 1.68
+            "//thirdparty/patches:boost-process-teminate-waitpid-nohang.patch",
         ],
     )