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/lib/sysroot.mk

@@ -34,7 +34,12 @@ BBBIN           = busybox-$(ARCH)
 BBHOME         ?= https://github.com/troglobit/busybox-builder/releases/download
 BBURL          ?= $(BBHOME)/$(BBVER)/$(BBBIN)
 
-_libs_src       = $(shell ldd $(FINITBIN) | grep -Eo '/[^ ]+')
+# glibc dlopen()s NSS modules at runtime, so ldd does not list them, but
+# without libnss_files getpwnam() cannot resolve users inside the chroot
+_libs_nss       = $(firstword $(wildcard /lib/$(ARCH)-linux-gnu/libnss_files.so.2 \
+                                         /usr/lib/$(ARCH)-linux-gnu/libnss_files.so.2 \
+                                         /lib64/libnss_files.so.2 /lib/libnss_files.so.2))
+_libs_src       = $(shell ldd $(FINITBIN) | grep -Eo '/[^ ]+') $(_libs_nss)
 libs            = $(foreach path,$(_libs_src),$(abspath $(DEST))$(path))
 
 all: $(libs) $(DEST)/bin/$(BBBIN)