Add short-iso-precise for journalctl output (#5884)

This adds a short-iso-precise option for journalctl output.  It is similar to
short-iso, but includes microseconds.

src/systemctl/systemctl.c

@@ -7001,7 +7001,8 @@ static void systemctl_help(void) {
                "     --root=PATH      Enable unit files in the specified root directory\n"
                "  -n --lines=INTEGER  Number of journal entries to show\n"
                "  -o --output=STRING  Change journal output mode (short, short-precise,\n"
-               "                             short-iso, short-full, short-monotonic, short-unix,\n"
+               "                             short-iso, short-iso-precise, short-full,\n"
+               "                             short-monotonic, short-unix,\n"
                "                             verbose, export, json, json-pretty, json-sse, cat)\n"
                "     --firmware-setup Tell the firmware to show the setup menu on next boot\n"
                "     --plain          Print unit dependencies as a list instead of a tree\n\n"