logind: Support logind.conf.d directories in the usual search paths

This makes it possible to drop in logind configuration snippets from a
package or other configuration management mechanism.

Add documentation to the header of /etc/logind.conf pointing the user at
/etc/logind.conf.d/*.conf.

Introduce a new helper, conf_parse_many, to parse configuration files in
a search path.

src/shared/conf-parser.c

@@ -27,6 +27,7 @@
 #include <netinet/ether.h>
 
 #include "conf-parser.h"
+#include "conf-files.h"
 #include "util.h"
 #include "macro.h"
 #include "strv.h"
@@ -430,6 +431,37 @@ int config_parse(const char *unit,
         return 0;
 }
 
+/* Parse each config file in the specified directories. */
+int config_parse_many(const char *conf_file,
+                      const char *conf_file_dirs,
+                      const char *sections,
+                      ConfigItemLookup lookup,
+                      const void *table,
+                      bool relaxed,
+                      void *userdata) {
+        _cleanup_strv_free_ char **files = NULL;
+        char **fn;
+        int r;
+
+        r = conf_files_list_nulstr(&files, ".conf", NULL, conf_file_dirs);
+        if (r < 0)
+                return r;
+
+        if (conf_file) {
+                r = config_parse(NULL, conf_file, NULL, sections, lookup, table, relaxed, false, true, userdata);
+                if (r < 0)
+                        return r;
+        }
+
+        STRV_FOREACH(fn, files) {
+                r = config_parse(NULL, *fn, NULL, sections, lookup, table, relaxed, false, true, userdata);
+                if (r < 0)
+                        return r;
+        }
+
+        return 0;
+}
+
 #define DEFINE_PARSER(type, vartype, conv_func)                         \
         int config_parse_##type(const char *unit,                       \
                                 const char *filename,                   \