Scripts called by the run script hook and signals

After #1657 (closed) which makes signals to be delivered only to the main thread the behavior is different for a script called by the main thread or called by a service thread. I propose to fix this in two phases:

• decide what behavior we want
• implement it in a consistent way

BTW the standard C system() function says in its manual that the caller ignores SIGINT and SIGQUIT, and blocks SIGCHLD. This means that with a caller not playing with signals SIGINT and SIGQUIT are handled by the shell, and SIGCHLD has its default (which is not to ignore it) before execve.