pid1: assume user namespaces are unavailable if we get -EINVAL from clone()

pid1: assume user namespaces are unavailable if we get -EINVAL from clone()

As reported in https://github.com/systemd/systemd/issues/35400,
on riscv64, with Linux version 6.6.51-linux4microchip+fpga-2024.09, we get:

[   10.063727] systemd[1]: systemd-modules-load.service: About to execute: /usr/lib/systemd/systemd-modules-load
[   10.071148] (journald)[104]: Failed to fork process (sd-mkuserns): Invalid argument

Fixes https://github.com/systemd/systemd/issues/35400.

'r' is used to make the repeated checks shorter. Without that, the long variable
name is distracting.