Fix #492: add per-service directories, systemd RuntimeDirectory style

A service that drops privileges cannot create its own PID file in
/run, root owns it.  Finit can create the file with pidfile-create,
but the daemon still cannot touch it to confirm a SIGHUP.

Five new settings, block format only: runtime-dir, state-dir,
cache-dir, logs-dir, and config-dir.  The value is a directory name,
resolved under /run, /var/lib, /var/cache, /var/log, and /etc,
respectively.  The directory is created before the service starts,
mode 0755 owned by user/group, and the full path is exported to the
process as RUNTIME_DIRECTORY, STATE_DIRECTORY, CACHE_DIRECTORY,
LOGS_DIRECTORY, and CONFIGURATION_DIRECTORY.  Mode and ownership are
asserted at creation only, a daemon may tighten them afterwards.

The runtime directory is removed when the unit stops, after any
exec-stop-post script, like systemd with RuntimeDirectoryPreserve=no.
A completed run/task counts as stopped unless remain-after-exit keeps
it up.  The other four persist across restarts.

These are the first settings with no legacy token: they are validated
by service_set_dir() and stored on the svc that service_register()
now returns.  systemd accepts a list of directories per setting; this
is a single name for now, widening later is compatible since
libconfuse accepts a bare value for a list option.

The test sysroot gains libnss_files.so.2, which ldd cannot see, glibc
dlopen()s it.  Without it getpwnam() fails inside the chroot, so
user/group settings never resolved and directory ownership could not
be tested.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/conf-dirs.sh

@@ -0,0 +1,80 @@
+#!/bin/sh
+# Verify the per-service directory settings: runtime-dir, state-dir,
+# cache-dir, logs-dir, and config-dir.  The directory is created before
+# the service starts, owned by the service user, and exported to the
+# environment.  The runtime directory is removed again when the service
+# stops, the others persist.
+set -eu
+
+TEST_DIR=$(dirname "$0")
+
+# shellcheck disable=SC2034
+BOOTSTRAP="service owned {
+    runlevel    = \"S12345\"
+    user        = \"daemon\"
+    group       = \"daemon\"
+    runtime-dir = \"owned\"
+    state-dir   = \"owned\"
+    pidfile     = \"/run/owned/serv.pid\"
+    command     = \"/sbin/serv -np -P /run/owned/serv.pid -i owned -e STATE_DIRECTORY:/var/lib/owned\"
+}
+task probe {
+    runlevel    = \"S12345\"
+    runtime-dir = \"probe\"
+    cache-dir   = \"probe\"
+    command     = \"/sbin/serv -h -e RUNTIME_DIRECTORY:/run/probe -e CACHE_DIRECTORY:/var/cache/probe\"
+}
+service escape {
+    runlevel    = \"S12345\"
+    runtime-dir = \"../escape\"
+    command     = \"serv -np -i escape\"
+}"
+
+# ls output is empty for an empty directory, so test -d instead
+assert_dir()
+{
+    assert "Directory $1 exists" "$(texec test -d "$1" && echo yes)" = "yes"
+}
+
+assert_nodir()
+{
+    assert "Directory $1 removed" "$(texec test -d "$1" || echo gone)" = "gone"
+}
+
+assert_owner()
+{
+    assert "$1 owned by $2" "$(texec stat -c %U:%G "$1")" = "$2"
+}
+
+# shellcheck source=/dev/null
+. "$TEST_DIR/lib/setup.sh"
+
+say 'Directory created before start, owned by the service user'
+retry 'assert_status owned running'
+assert_dir /run/owned
+assert_owner /run/owned daemon:daemon
+assert_dir /var/lib/owned
+assert_owner /var/lib/owned daemon:daemon
+
+say 'The paths are exported to the process environment; both commands'
+say 'verify their own with serv -e, and refuse to run on a mismatch'
+
+say 'A completed task no longer holds its runtime directory'
+retry 'assert_status probe done'
+assert_nodir /run/probe
+assert_dir /var/cache/probe
+
+say 'Stopping the service removes the runtime directory, state persists'
+run "initctl stop owned"
+retry 'assert_status owned stopped'
+assert_nodir /run/owned
+assert_dir /var/lib/owned
+
+say 'Starting again recreates it'
+run "initctl start owned"
+retry 'assert_status owned running'
+assert_dir /run/owned
+
+say 'A path escaping the base directory is refused, service still runs'
+retry 'assert_status escape running'
+assert_nodir /escape