core/load-fragment: don't treat "; ;" as "/usr/bin/;"

We had a special test case that the second semicolon would be interpreted
as an executable name. We would then try to find the executable and rely
on ";" not being found to cause ENOEXEC to be returned. I think that's just
crazy. Let's treat the second semicolon as a separator and ignore the
whole thing as we would whitespace.

src/core/load-fragment.c

@@ -721,6 +721,12 @@ int config_parse_exec(
                 if (r <= 0)
                         return 0;
 
+                /* A lone ";" is a separator. Let's make sure we don't treat it as an executable name. */
+                if (streq(firstword, ";")) {
+                        semicolon = true;
+                        continue;
+                }
+
                 f = firstword;
                 for (;;) {
                         /* We accept an absolute path as first argument.  If it's prefixed with - and the path doesn't

src/test/test-load-fragment.c

@@ -202,12 +202,11 @@ static void test_config_parse_exec(void) {
                               "-@/RValue argv0 r1 ; ; "
                               "/goo/goo boo",
                               &c, u);
-        assert_se(r == -ENOEXEC);
+        assert_se(r >= 0);
         c1 = c1->command_next;
         check_execcommand(c1, "/RValue", "argv0", "r1", NULL, true);
-
-        /* second command fails because the executable name is ";" */
-        assert_se(c1->command_next == NULL);
+        c1 = c1->command_next;
+        check_execcommand(c1, "/goo/goo", "/goo/goo", "boo", NULL, false);
 
         log_info("/* trailing semicolon */");
         r = config_parse_exec(NULL, "fake", 5, "section", 1,