Remove all calls to exit(3) from libavrdude library functions

[dl8dtl]

(Previously, this had been marked as a "task" on Savannah, with nobody actually backing it.)
Quite some time ago, all the backend functionality of AVRDUDE had been moved out into libavrdude. The idea behind that was that the current src/main.c just describes one possible frontend, so it could be e.g. accompanied by a kind of GUI frontend as well.
However, some of the historic implementations gratuitously call exit() in case of a failure. This is fatal for a GUI application, which rather would prefer to give the user a kind of graphical response of the failure. Thus, call calls to exit() need to be converted into returning (and handling) an error code instead.

[mcuee]

Interesting background for the libavrdude.

In order for this idea to work, #952 (#954 is the same for Windows) needs to be fixed. The main problem is Windows.

• Unable to build libavrdude dynamic and static library with CMake buildsystem #952
• Windows shared library build problem under both VS2019 and MSYS2 #954

[stefanrueger]

Converting exit(n) into return -n is relatively hard, as this may involve returning through a number of nested calls on the stack. I wonder whether setjmp()/longjmp() can provide a useful interface for the (weaker) aim of keeping the GUI alive so it can handle any problems gracefully. Converting exit() to longjmp() can be done with relative ease.

[mcuee]

Converting exit(n) into return -n is relatively hard, as this may involve returning through a number of nested calls on the stack. I wonder whether setjmp()/longjmp() can provide a useful interface for the (weaker) aim of keeping the GUI alive so it can handle any problems gracefully. Converting exit() to longjmp() can be done with relative ease.

Any new ideas about this issue?