Use hard-coded FD for stdio in subprocess

[fantix]

Use FD=0/1/2 for subprocess(stdin=None, stdout=None, stderr=None). This aligns uvloop with the same behavior in asyncio - when stdin, stdout or stderr is None, the subprocess will use FD 0, 1 or 2 now instead of sys.stdin, sys.stdout or sys.stderr.

asyncio eventually calls this to create subprocess, where p2cread, c2pwrite or errwrite are -1 and the current FD 0, 1 and 2 are left untouched to eventually call execv().

Fixes #136

• Add test
• Add description
• Fix commit message

[1st1]

This aligns uvloop with the same behavior in asyncio - when stdin, stdout or stderr is None, the subprocess will use FD 0, 1 or 2 now instead of sys.stdin, sys.stdout or sys.stderr.

This is news to me...

[fantix]

Well yeah - looks like it's not strictly FD 0/1/2 but rather a simple fork() followed by an execv(), without modifying FD 0/1/2 by dup2() or ioctl() (as part of set_inheritable()).