Fix a SEGFAULT that was hidden by our waitpid() calls. (#422)

When a subprocess terminates with a successful return code, it might still
have been terminated by a signal, one signal would be SIGSEGV. Arrange our
code to report when that happens.

This happened in initialisation of the streaming module when trying to call
setvbuf on a un-assigned file descriptor. This is fixed in follow.c when
preparing the call.