The sigaction handler called map on an option, creating a pointer to a
move value. This in turned caused UB for signal handlers. Avoid using
pointers directly, and instead prefer references.

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

I really really wish I could actually test this on redox. All I know is: it compiles