A JobId is not supposed to convert to other types.
Since this type is defined as NonZeroU32 (which cannot be -1), we need to
add some conversion functions to match the C++ behavior.
Overall, it would have been better to keep using the C++ type.
Everything but signal handlers has been changed to use `Signal` instead of
`c_int` or `i32` signal values.
Event handlers are using `usize` to match C++, at least for now.
More ugliness with types that cxx bridge can't recognize as being POD. Using
pointers to get/set `termios` values with an assert to make sure we're using
identical definitions on both sides (in cpp from the system headers and in rust
from the libc crate as exported).
I don't know why cxx bridge doesn't allow `SharedPtr<OpaqueRustType>` but we can
work around it in C++ by converting a `Box<T>` to a `shared_ptr<T>` then convert
it back when it needs to be destructed. I can't find a clean way of doing it
from the cxx bridge wrapper so for now it needs to be done manually in the C++
code.
Types/values that are drop-in ready over ffi are renamed to match the old cpp
names but for types that now differ due to ffi difficulties I've left the `_ffi`
in the function names to indicate that this isn't the "correct" way of using the
types/methods.
