conf: adopt the systemd semantics for per-service directories

Aaron Andersen points out in the #492 discussion that the *Directory
settings carry more contract than create-and-chown: per-directory
modes, specific ownership rules, and cleanup toggles.  Without them
config-dir was chowned to the service user, which systemd never does,
an existing directory with drifted ownership was left wrong, and the
runtime directory could not survive a restart.

Now matching systemd.exec(5), and where the man page is vague, the
code in setup_exec_directory():

  - each directory takes a matching -mode key, octal with the leading
    zero, default 0755.  The mode of the named directory is locked
    down again on every start, also when it already exists
  - config-dir is created but never chowned
  - the contents of an existing directory are left alone as long as
    the owner is right; on drift everything under it is chowned back
  - runtime-dir-preserve = no | restart | yes maps
    RuntimeDirectoryPreserve=.  A service still qualified to run when
    the runtime directory would be removed is restarting, not
    stopping, which is what svc_enabled() answers

The dir mechanics move to mksubsysd(), taking resolved ids, with
mksubsys() reduced to a name-resolving wrapper for the dbus plugin.
The child resolves uid/gid once for both directory setup and
privilege drop.

The symlink form, RuntimeDirectory=foo:bar, is not adopted.

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

src/svc.h

@@ -201,11 +201,17 @@ typedef struct svc {
 
 	/* Directories set up for the service, block format only, the
 	 * name is resolved under a fixed base, e.g. /run/NAME */
+#define NUM_SVCDIRS 5
 	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];
+	mode_t	       dir_mode[NUM_SVCDIRS];
+#define SVC_DIR_PRESERVE_NO      0	/* remove runtime-dir on stop  */
+#define SVC_DIR_PRESERVE_RESTART 1	/* keep it across restarts     */
+#define SVC_DIR_PRESERVE_YES     2	/* never remove it             */
+	char	       dir_preserve;
 
 	/* Command, arguments and service description */
 	char	       cmd[MAX_CMD_LEN];