Nagios checks NOTE and EXITSTATUS are redundant

[Baron Schwartz]

There's an opportunity for bugs to be introduced in case I set the exit status to one thing and the note's prefix to another. Also, using exit at the end of main() makes it hard to do integration testing.

To resolve this, I will create a new function that sets the exit status based on the prefix of the note. When the note doesn't exist, the exit status will be unknown. This function will be called when the script is executed, but not when main() is run as a function from a sourcing file.