shared/verbs: add command_print_verb_help()

A helper to print the help of a single verb, derived entirely from the
data in the SYSTEMD_VERBS and SYSTEMD_OPTIONS sections: the synopsis is
built from the verb name and argspec, the one-line verb help serves as
the abstract, the options come from the verb's option namespace, and the
man page links from the command the verb belongs to. A flag allows
suppressing the man page links for callers which append custom sections.

src/test/test-verbs.c

@@ -114,4 +114,13 @@ TEST(introspect_cli_verb_options) {
                                   SD_JSON_FORMAT_OFF));
 }
 
+TEST(command_print_verb_help) {
+        ASSERT_OK(_command_print_verb_help(introspect_verbs, introspect_verbs + ELEMENTSOF(introspect_verbs) - 1,
+                                           introspect_options, introspect_options + ELEMENTSOF(introspect_options) - 1,
+                                           "alpha"));
+        ASSERT_OK(_command_print_verb_help(introspect_verbs, introspect_verbs + ELEMENTSOF(introspect_verbs) - 1,
+                                           introspect_options, introspect_options + ELEMENTSOF(introspect_options) - 1,
+                                           "beta"));
+}
+
 DEFINE_TEST_MAIN(LOG_INFO);