installer/resources/i2prouter: various bugfixes

- Explicitly specify shell when RUN_AS_USER is set. This allows I2P to start
  under limited accounts that do not have a defined shell.
- When installing as a service, point out that I2P_CONFIG_DIR may need to be
  set. I2P_CONFIG_DIR defaults to $HOME/.i2p for the user that installed I2P
  and when running as a service this may not be desired.
  (e.g., RUN_AS_USER=i2p & I2P_CONFIG_DIR=/root/.i2p)

(The RUN_AS_USER shell problem was spotted at https://github.com/mattias-ohlsson/i2p/blob/master/i2p.spec)