src/Makefile: Add dependency on the Makefile.
main.lua, snabbswitch.c: Add command-line arguments to snabbswitch.

    Usage: snabbswitch [options]
    Available options are:
    -e chunk     Execute string 'chunk'.
    -l name      Require library 'name'.
    -d           Debug unhandled errors with the Lua interactive debugger.
    -jdump file  Trace JIT decisions to 'file'. (Requires LuaJIT jit.* library.)

This means we can tell snabbswitch what we want it to do using the
command-line, instead of (as I have) always hacking selftest.lua to do
whatever it is I'm interested in today.

Default behavior is to assume "-l selftest" (i.e. backwards compatible).