fileio: accept FILE* in addition to path in parse_env_file()

Most our other parsing functions do this, let's do this here too,
internally we accept that anyway. Also, the closely related
load_env_file() and load_env_file_pairs() also do this, so let's be
systematic.

src/basic/util.c

@@ -264,7 +264,7 @@ int container_get_leader(const char *machine, pid_t *pid) {
                 return -EINVAL;
 
         p = strjoina("/run/systemd/machines/", machine);
-        r = parse_env_file(p, NEWLINE, "LEADER", &s, "CLASS", &class, NULL);
+        r = parse_env_file(NULL, p, NEWLINE, "LEADER", &s, "CLASS", &class, NULL);
         if (r == -ENOENT)
                 return -EHOSTDOWN;
         if (r < 0)