machined: when reading os-release file, join PID namespace too

machined: when reading os-release file, join PID namespace too

This is required for /proc/self/fd/xyz to work, but that's what we need
to convert the O_PATH fd returned by chase_symlinks() back to a regular
file fd. Hence, let's do the joining of the namespaces fully and
correctly, by doing fork()+setns()+fork() with the PID and fs
namespaces.

This makes use of the new namespace_fork() helper we just added.

Fixes: #10549