On systems without an i2psvc group, the postinstall would fail. This change
will add the group if it doesn't exist. Also adding "||true" to the usermod
command in case the command returns a non-zero status.
The amount of RAM that I2P is limited to can be adjusted using debconf. The
value in /etc/i2p/wrapper.config is stored in the debconf db and is written to
/etc/default/i2p.
