Make label prefixing more strict

This patch unconditionally applies the labeling of sessions and thereby
removes the most common use case of 'Child_policy::filter_session_args'.
Furthermore, the patch removes an ambiguity of the session labels of
sessions created by the parent of behalf of its child, e.g., the PD
session created as part of 'Child' now has the label "<child-name>"
whereas an unlabeled PD-session request originating from the child
has the label "<child-name> -> ". This way, the routing-policy of
'Child_policy::resolve_session_request' can differentiate both cases.

As a consequence, the stricter labeling must now be considered wherever
a precise label was specified as a key for a session route or a server-
side policy selection. The simplest way to adapt those cases is to use a
'label_prefix' instead of the 'label' attribute. Alternatively, the
'label' attribute may used by appending " -> " (note the whitespace).

Fixes #2171

repos/dde_rump/run/rump_ext2.run

@@ -60,7 +60,7 @@ append config {
 	<start name="rump_fs">
 		<resource name="RAM" quantum="8M" />
 		<provides><service name="File_system"/></provides>
-		<config fs="ext2fs"><policy label="test-libc_vfs" root="/" writeable="yes"/></config>
+		<config fs="ext2fs"><policy label_prefix="test-libc_vfs" root="/" writeable="yes"/></config>
 	</start>
 	<start name="test-libc_vfs">
 		<resource name="RAM" quantum="4M"/>

repos/dde_rump/run/rump_iso.run

@@ -54,7 +54,7 @@ append config {
 	<start name="rump_fs">
 		<resource name="RAM" quantum="8M" />
 		<provides><service name="File_system"/></provides>
-		<config fs="cd9660"><policy label="fs_rom" root="/" writeable="no"/></config>
+		<config fs="cd9660"><policy label_prefix="fs_rom" root="/" writeable="no"/></config>
 	</start>
 	<start name="fs_rom">
 		<resource name="RAM" quantum="2M"/>