SF patch #489173: Make os.spawnv not block the interpreter, from

SF patch #489173:  Make os.spawnv not block the interpreter, from
Anthony Roach.
Release the global interpreter lock around platform spawn calls.
Bugfix candidate?  Hard to say; I favor "yes, bugfix".
These clearly *should* have been releasing the GIL all along, if for no
other reason than compatibility with the similar os.system().  But it's
possible some program out there is (a) multithreaded, (b) calling a spawn
function with P_WAIT, and (c) relying on the spawn call to block all their
threads until the spawned program completes.  I think it's very unlikely
anyone is doing that on purpose, but someone may be doing so by accident.