create: refactor arch detection to run_fork_with_timeout

create: refactor arch detection to run_fork_with_timeout

Swap out our own fork/waitpid code with run_fork_with_timeout, which
not only allows to return arbitrary results from the called method
but also has a timeout configured, which prevents that a creation
hangs forever (= next reboot).

As we can now return more than with an exit code number we do not
return the ELF class but the detected architecture directly and pull
the fallback code into this method.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>