narrow Subprocess.__init__ Popen except to real Popen failures

[HrachShah]

narrow Subprocess.init Popen except to real Popen failures

The bare 'except:' wrapped subprocess.Popen() and a per-init pipe FD
cleanup loop. The narrow set covers the actual failures of Popen():
OSError (FileNotFoundError when the executable isn't found,
PermissionError when it isn't executable, OSError for other fork/exec
failures), ValueError (embedded null byte in executable path),
TypeError (unknown kwarg), and subprocess.SubprocessError (parent of
all subprocess-raised errors added in Python 3.3; covers the cases where
Popen detects an invalid argument combination internally).

The bare except was also catching BaseException including KeyboardInterrupt
and SystemExit — a Ctrl-C during the Popen call now propagates
immediately to the asyncio task that owns the Subprocess instead of
being swallowed and re-raised after the FD cleanup loop.