Suppress backtrace in reaction to Ctrl-C

[fingolfin]

It would be nice if pressing ctrl-c would not print a backtrace. Otherwise, when doing make -j8 and then aborting the build, I get a ton of pointless backtraces.

[fingolfin]

It might be possible to do this by installing our own signal handler for SIGINT, e.g. using https://github.com/LuaDist/lua-signal or https://luaposix.github.io/luaposix/modules/posix.signal.html

Might have to do something else for LuaJIT. Perhaps we can use this example: http://stackoverflow.com/questions/19352389. Note that we only want to exit in the signal handler, so hopefully the caveats listed there do not apply to us.