Make child process InheritFd by default

[m13253]

In the current design of std::io::process, three sets of pipes are created for the child process: std{in,out,err}.

I submit this issue here, to ask for support to make child process InheritFd by default in the new design, that is, use the parent process's std{in,out,err} instead of pipes.

Reasons:

• Child processes are InheritFd by default both on POSIX and Windows, with native API. Pipes can be explicitly set up.
• Pipes are not used so often that it should be the default. At least not all std{in,out,err} are used at the same time that often.
• It is potential that the programmer forgets to set InheritFd when pipes are unneeded. Which may lead to the child process being locked up, when the child process writes out too much thing but the parent process does not read anything from the pipe.
• If the parent process terminates, the child process will crash with "broken pipe" when trying to read or write.
• Inheritance of file descriptors other than 0, 1, 2 are not supported in current design. However, it is useful in many applications (apt-get uses fd 4 to exchange status data between processes), where sockets are inappropriate.
• Backwards compatibility is not a problem, since we are redesigning std::io completely these days.

[alexcrichton]

Could you take a look at #579 as well? I believe it takes this into account. I'm going to close in favor of that RFC, but thanks for the issue!