Sorry for the bad English.
You might want to take a look at libfuzzer (http://llvm.org/docs/LibFuzzer.html), which you can use to test library functions. What you do is implement a function called LLVMFuzzerTestOneInput that should call whatever library function you want to test. One of the parameters is a uint8_t array that can be transformed into whatever kind of parameter types the library function expects. Then libfuzzer provides the main function that will generate the data and call the LLVMFuzzerTestOneInput function, repeating the process until there is a crash.
The one downside is that you need to recompile the code (the library) you are trying to fuzz.
It uses genetic algorithms to spawn agents that call an API with all kinds of arguments and reproduce when it finds unique code-paths.
Before anyone asks: the code is far from complete, it's really more a fiddle than anything.
It seems that wrapping select()/read()/write() to have random readability/writability/closing seeded by a fuzz input might exercise those code paths. I'm yet to implement it though - be interested to hear of prior work. Getting the fuzz input corpus to correlate to program actions might be problematic.
But you are right, that the configuration can be part of the fuzzed input. It should be possible to take part of the data fed into Irssi by AFL and use that as the config file and then use the rest as the network traffic.
It might be a good idea to check the parser for bugs but these aren't real security issues.
Seems in my mind to be a nice complement to achieving code-coverage with testing i.e. whereas unit/integration testing might test the various code paths with a few good/bad values, this then throws every possible input value at them to see what breaks.
To put it another way - PIT runs your unit tests against automatically modified versions of your application code. When the application code changes, it should produce different results and cause the unit tests to fail. If a unit test does not fail in this situation, it may indicate an issue with the test suite.