Echo can interpret operand as an option and checking every variable to
be echoed is troublesome while with printf, if the format specifier is
present before the operand, printing as string can be enforced.
- Find PGP keys using the same methods as other scripts;
- Lower threshold to 30 days by default;
- Add environment variable to set threshold;
- Add colors to distinguish expired from expires soon; and
- Add days until key expiration when it is below threshold.
