[Libguestfs] [PATCH] Add safe wrapper around waitpid which deals with EINTR correctly.

As Eric Blake noted in: https://www.redhat.com/archives/libguestfs/2016-April/msg00154.html libguestfs doesn't correctly handle the case where waitpid receives a SIGCHLD signal and the main program has registered a non-restartable signal handler. In this case waitpid would return -EINTR and we would print an error, but actually we should retry this case. This adds two new internal functions, one for handling waitpid and printing (real) errors from it, the other for just waiting for a process where we don't particularly care about errors. Rich.