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>

src/svc.h

@@ -199,6 +199,14 @@ typedef struct svc {
 	int		       num_supgroups;
 	char	       capabilities[MAX_CMD_LEN];
 
+	/* Directories set up for the service, block format only, the
+	 * name is resolved under a fixed base, e.g. /run/NAME */
+	char	       runtime_dir[MAX_ARG_LEN];
+	char	       state_dir[MAX_ARG_LEN];
+	char	       cache_dir[MAX_ARG_LEN];
+	char	       logs_dir[MAX_ARG_LEN];
+	char	       config_dir[MAX_ARG_LEN];
+
 	/* Command, arguments and service description */
 	char	       cmd[MAX_CMD_LEN];
 	char	       args[MAX_NUM_SVC_ARGS][MAX_CMD_LEN];