`posix_spawn` fails starting in glibc-2.38 about shadow

[stevenengler]

It might also be worth fixing the "Dropped without calling safely_drop" errors, which cause panics in debug builds when unexpected flags are passed to the clone syscall. There are two return calls in clone_internal that don't properly drop the descriptor table's RootedRc before returning. (Also these error messages should probably say "explicit_drop", not "safely_drop".)

[sporksmith]

Added a test for this first. Over the course of which I learned:

• We currently have a bug s.t. the signal handlers always get set to default in a child process. Whoops.
• Experimentally it looks like CLONE_CLEAR_SIGHAND is ignored in a clone syscall; it only works in clone3. My guess is that the clone syscall only supports flags that fit in 32 bits; (clone3 explicitly adds support for 64-bit wide flags).

With those details sorted out, actually implementing the flag should be pretty simple.