Error handling in eConsoleAppContainer

There are some problems with error handling in eConsoleAppContainer.

1. If the execvp() call in bidirpipe() returns (e.g. fails with ENOENT or EPERM), the child process prints "[eConsoleAppContainer] Finished <cmdname>" to the debug log when it was actually not possible to run the command, and exits the child process with status 0, when it should exit the child with non-zero status. Exiting with status 127 would be compatible with the status returned from "sh -c" when the command isn't found.
2. If the child is terminated with a signal, the appClosed() PSignal1 is invoked with a retval of 0, unless the process was terminated by a SIGKILL that was sent by eConsoleAppContainer::kill(). If the child is terminated by a SIGKILL from any other source, retval is 0. Similarly retval is 0 if the child is terminated by SIGINT, even if the SIGINT was sent by eConsoleAppContainer::sendCtrlC().

I propose that if the execvp() call in bidirpipe() returns, the message printed to the debug lok should be something like "[eConsoleAppContainer] <cmdname> failed to execute: <perror string>", and the call of _exit() shoud be changed from _exit(0) to _exit(127).

I think that the retval argument to the appClosed() PSignal1 should be the child's exit status if the child exited normally, and -signalvalue if the child was terminated by a signal. I have run through most of the uses of eConsoleAppContainer in the Beyonwiz enigma2 code and I can't see any problem with this change (retval is mostly ignored or tested for being non-zero). The only issue I see is if there is code that explicitly tests for retval == -1 to detect termination of the child by eConsoleAppContainer::kill().