shared/options: add OPTION_COMMON_{SYSTEM,USER}

We have different help strings for --user/--system in different places, so this
only covers a subset of --system/--user instances. But this particular help
seems to be the most widely used.

(In a few cases, the help string is fixed: it should be "system mode", not
"per-system mode".)

src/shared/options.h

@@ -124,6 +124,12 @@ typedef struct Option {
 #define OPTION_COMMON_MACHINE                                           \
         OPTION('M', "machine", "CONTAINER", "Operate on local container")
 
+#define OPTION_COMMON_SYSTEM                                            \
+        OPTION_LONG("system", NULL, "Operate in system mode")
+
+#define OPTION_COMMON_USER                                              \
+        OPTION_LONG("user", NULL, "Operate in per-user mode")
+
 #define OPTION_COMMON_JSON                                              \
         OPTION_LONG("json", "FORMAT", "Generate JSON output (pretty, short, or off)")