Allow users to change timeout of cryptroot-unlock with an new option `-t`

Hi :)

I've been using cryptroot-unlock for a while, and sometimes my hosts are quite slow. So the script exists with a non-zero code because the timeout was reached. And a few seconds later, the unlocking succeeds. So I thought it might be helpful to allow users to specify how long they want to wait. The old default of 10s stays the same, though.

I currently only added this -t option + a little help page with -h. But I could also add other options, if you see any fit. Please let me know what you think!