MINOR: mworker: readapt program support in mworker_catch_sigchld

MINOR: mworker: readapt program support in mworker_catch_sigchld

This patch is a part of series to reintroduce the program support in the new
master-worker architecture.

We just only launch and stop external programs and there is no any
communication between the master process and the started program binary. So,
ipc_fd[0] and ipc_fd[1] are not used and kept as -1 for programs processes. Due
to this, no need for the exiting program process to call fd_delete on this
fds. Otherwise, this will trigger a BUG_ON.