Fix return code for spawnvpe/waitpid.

[elfpipe]

1. Redo return code for spawnees, so that WIFEXITED() tests for a flag (0x80000000)
2. Fix entry code for spawnees, so that group id is supplied though the entry data parameter. This is because, apparently, opening usergroup.library inside entry code makes it crash.
3. Change number of arguments for insertSpawnedChildren().

This update will enable cmake to build a valid makefile project.