base: apply routing policy to environment sessions

This patch changes the child-construction procedure to allow the routing
of environment sessions to arbitrary servers, not only to the parent.
In particular, it restores the ability to route the LOG session of the
child to a LOG service provided by a child of init. In principle, it
becomes possible to also route the immediate child's PD, CPU, and RAM
environment sessions in arbitrary ways, which simplifies scenarios that
intercept those sessions, e.g., the CPU sampler.

Note that the latter ability should be used with great caution because
init needs to interact with these sessions to create/destruct the child.
Normally, the sessions are provided by the parent. So init is safe at
all times. If they are routed to a child however, init will naturally
become dependent on this particular child. For the LOG session, this is
actually not a problem because even though the parent creates the LOG
session as part of the child's environment, it never interacts with the
session directly.

Fixes #2197

repos/base-linux/src/lib/base/child_process.cc

@@ -63,12 +63,11 @@ Child::Process::Process(Dataspace_capability  elf_ds,
                         Pd_session_capability pd_cap,
                         Pd_session           &pd,
                         Ram_session          &ram,
-                        Initial_thread_base  &initial_thread,
+                        Initial_thread_base  &,
                         Region_map           &local_rm,
                         Region_map           &remote_rm,
                         Parent_capability     parent_cap)
 :
-	initial_thread(initial_thread),
 	loaded_executable(elf_ds, ldso_ds, ram, local_rm, remote_rm, parent_cap)
 {
 	/* skip loading when called during fork */