This should avoid strange behavior due to races when the parent has resumed but the child is still running the exec* code in libc, which mainly manifests itself when running at low speed.
We also change to signaling the child's completion with SIGALRM, and setting an extra alarm in the parent in case the child doesn't actually do it.
Dmake gets confused if you use over-15-character file names, so they needed to be shortened to get it to work. While I was at it, I also switched _ to . in the filenames so they're fully ProDOS compatible.
The shell/hush_test directory (not needed to build the code) still uses ProDOS-incompatible file names.
