Streamline file descriptor handling during spawning.

 * Leak of the output pipe file descriptors if control pipe can't be opened.
 * Leak of posix file actions memory if the control pipes can't be created.
 * Descriptors leak of unrelated tasks into spawned task (linux, win).
 * Windows code improperly inheriting the control file descriptors.
 * A race condition in the control channel parsing on windows resulting in
   a potential double-free.

rdar://56800263

Author: vlm