vmspawn: keep parsing options after the first argument

vmspawn's positional arguments are extra kernel command line arguments
rather than a command to execute, so unlike nspawn or run it has nothing
to shield from the option parser: a kernel command line argument never
begins with a dash. Stopping at the first one only means an option that
follows one is silently taken for another kernel command line argument,
with no diagnostic:

    systemd-vmspawn -i image.raw console=ttyS0 --set-credential=foo:bar

boots with "--set-credential=foo:bar" appended to the kernel command
line instead of setting a credential.

The stop was right when it was written. In 9de3cc1484, vmspawn's first
commit, the trailing arguments were appended to the QEMU command line
directly, so they really were options that had to reach QEMU unparsed.
4291f4461e turned them into extra kernel command line arguments passed
through SMBIOS three weeks later, and updated the man page to say so,
but left the "+" in the optstring behind; 4c778c51c0 and 5ff0ccaf8b then
carried it into the new option parser as
OPTION_PARSER_STOP_AT_FIRST_NONOPTION.

Use the default parser mode, which permutes options ahead of the
arguments; "--" still ends option parsing for an argument that does look
like an option. test-options gains the case this turns on, an option
after a positional argument in the default mode, beside the existing one
for the mode vmspawn no longer uses.

src/test/test-options.c

@@ -196,6 +196,20 @@ TEST(option_parse) {
                               OPTION_PARSER_STOP_AT_FIRST_NONOPTION,
                               NULL);
 
+        test_option_parse_one(STRV_MAKE("arg0",
+                                        "string1",
+                                        "--help",
+                                        "string2"),
+                              options,
+                              (Entry[]) {
+                                      { "help" },
+                                      {}
+                              },
+                              STRV_MAKE("string1",
+                                        "string2"),
+                              OPTION_PARSER_NORMAL,
+                              NULL);
+
         test_option_parse_one(STRV_MAKE("arg0",
                                         "-h"),
                               options,

src/vmspawn/vmspawn.c

@@ -344,7 +344,11 @@ static int parse_argv(int argc, char *argv[]) {
         assert(argc >= 0);
         assert(argv);
 
-        OptionParser opts = { argc, argv, OPTION_PARSER_STOP_AT_FIRST_NONOPTION };
+        /* Our positional arguments are kernel command line arguments rather than a command to
+         * execute, and those never begin with a dash, so there's no reason to stop looking for
+         * options at the first of them. "--" still ends option parsing, for the rare argument that
+         * does look like an option. */
+        OptionParser opts = { argc, argv };
 
         FOREACH_OPTION_OR_RETURN(c, &opts)
                 switch (c) {