Make the pid parameter default to the PID of the current process.
Change the exit code for invalid arguments to a value in the user
defined range (see https://tldp.org/LDP/abs/html/exitcodes.html). This
may be considered a breaking change, but since the only applications
that currently use this script don't rely on this code, I don't think
we need to bump the major version for this.
The daemon and scheduler will need rpid to get the PID of the login
process that they are both descended from in order to create a unique
fifo in order to not conflict with other daemons and schedulers in
other sessions.