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/service.h

@@ -27,6 +27,22 @@
 
 #include "svc.h"
 
+/*
+ * systemd-style per-service directories, block format only: the .conf
+ * key, the base the name resolves under, and the environment variable
+ * the path is exported as.  Shared by service.c and conf.c so adding
+ * one means touching the table and svc.h alone.
+ */
+struct svcdir {
+	const char *key;
+	const char *base;
+	const char *env;
+	size_t      off;	/* offsetof() in svc_t */
+};
+#define NUM_SVCDIRS 5
+extern const struct svcdir svcdirs[NUM_SVCDIRS];
+
+int	  service_set_dir	 (svc_t *svc, const struct svcdir *sd, const char *name);
 svc_t	 *service_register	 (int type, char *line, struct rlimit rlimit[], char *file);
 void      service_unregister     (svc_t *svc);