log: change log_xyz_errno() calls to always return the negative error

This enables us to write things like this:

    int open_some_file(void) {

            fd = open("/dev/foobar", O_RDWR|O_CLOEXEC);
            if (fd < 0)
                    return log_error_errno(errno, "Failed to reboot: %m");

            return fd;
    }

Which is function that returns -errno on failure, as well as printing an
error message, all in one line.