Notes about systemd unit files for bip.

Distro specific commandline configuration is provided by installing a script
named. A default script named bip_env.sh is provided.

This should write /run/sysconfig/bip based on configuration
information such as in /etc/sysconfig/bip or /etc/defaults/bip. It is run once
by bip-config.service.

Distro specific path for bip_env.sh script must be set using a
bip-config.service.d/distrib.conf unit file drop-in:
[Service]
ExecStart=/path/to/bip_env.sh

bip_env.sh try some default paths for the default configuration file: either
/etc/default/bip or /etc/sysconfig/bip.
Optionaly, the default configuration file path can be set too:
[Service]
ExecStart=/path/to/bip_env.sh
Environment=BIP_DEFAULT_CONFIG='/path/to/default/bip'
