util: Fix assertion in split() on missing '

When parsing a unit with a trailing slash after an escaped line break, like

  ExecStart=/bin/echo 'foo \
    bar'

the split() function (through config_parse()) asserted and crashed pid 1:

  Assertion 'current[*l + 1] == quotechars[0]' failed at ../src/shared/util.c:583, function split(). Aborting.

Fix this by returning an error in this case ("trailing garbage").

Add corresponding test case. Also fix the missing "unit" argument of
config_parse_exec() in the comment.

https://launchpad.net/bugs/1447243

src/test/test-unit-file.c

@@ -91,6 +91,7 @@ static void check_execcommand(ExecCommand *c,
 
 static void test_config_parse_exec(void) {
         /* int config_parse_exec(
+                 const char *unit,
                  const char *filename,
                  unsigned line,
                  const char *section,
@@ -302,6 +303,20 @@ static void test_config_parse_exec(void) {
         assert_se(r == 0);
         assert_se(c1->command_next == NULL);
 
+        log_info("/* missing ending ' */");
+        r = config_parse_exec(NULL, "fake", 4, "section", 1,
+                              "LValue", 0, "/path 'foo",
+                              &c, NULL);
+        assert_se(r == 0);
+        assert_se(c1->command_next == NULL);
+
+        log_info("/* missing ending ' with trailing backslash */");
+        r = config_parse_exec(NULL, "fake", 4, "section", 1,
+                              "LValue", 0, "/path 'foo\\",
+                              &c, NULL);
+        assert_se(r == 0);
+        assert_se(c1->command_next == NULL);
+
         exec_command_free_list(c);
 }