Serialdump now exits cleanly on ctrl+c

While we're at it that the make command fails where it is good to fail (parallel programming...) I thought it would be nice when make does not fail when the serialdump is terminated by the user.

When using the command `make login` the serialdump program starts and executes in an endless loop. The program is terminated by the user by entering <kbd>ctrl</kbd>+<kbd>c</kbd>. Because this input is not handled by the program it exits with an error code that make interprets as the build has failed. I think make should not fail here because this is the way it is intended to be used.

In this PR serialdump exits with code 0 and signals to make that everything is fine.

Someone (@comawill) should test this on OSX and provide the corresponding binary.