iodine: improve password handling (#58806)

Before this change, only passwords not containing shell metacharacters could be
used, and because the password was passed as a command-line argument, local
users could (in a very small window of time) record the password and (in an
indefinity window of time) record the length of the password.

We also use the opportunity to add a call to `exec` in the systemd start
script, so that no shell needs to hang around waiting for iodine to stop.