Process should support the child outliving the parent

[alexcrichton]

Current, Process invokes wait in its destructor to prevent leaking the child process for a long-running parent (see #13854 for discussion and more info). This does not support the use case of a child outliving the parent (such as creating a daemon).

A new method should be added to the Process type, perhaps detach, which consumes the Process and cancels the call to wait in the destructor. Something along the lines of:

// Forgets this process, allowing it to outlive the parent
//
// This function will forcefully prevent calling `wait()` on the child
// process in the destructor, allowing the child to outlive the 
// parent. Note that this operation can easily lead to leaking the
// resources of the child process, so care must be taken when
// invoking this method.
pub fn detach(self) { /* ... */ }

[glandium]

There's already a detach() function on Command, which does something related but different. There might be risk for confusion if the same name is used.