[Bug 231933] bc hanging and leaving zombie of dc when being called by process with SIGCHLD blocked

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231933

            Bug ID: 231933
           Summary: bc hanging and leaving zombie of dc when being called
                    by process with SIGCHLD blocked
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: vladimirv at ixsystems.com

Created attachment 197768
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=197768&action=edit
Patch that fixes described issue

In our system (http://www.freenas.org/) uwsgi application server calls a python
program that calls a shell script that calls a shell script that calls bc and
it hangs leaving us with hanging process tree like this:

```
3135   |-- /usr/local/bin/uwsgi <...long text cut>
4435   | `-- /usr/local/bin/uwsgi <...long text cut>
8952   |   |-- /bin/sh /usr/sbin/service smartd-daemon status
9087   |   | `-- /bin/sh /usr/sbin/service smartd-daemon status
9088   |   |   `-- /bin/sh /usr/sbin/service smartd-daemon status
9090   |   |     `-- /usr/bin/bc
9091   |   |       `-- <defunct>
```

This happens because bc relies on SIGCHLD to determine that dc has completed,
wait for it and exit on it's own. SIGCHLD is blocked by uwsgi and this blocking
is inherited by each it's subprocess. Our shell scripts are rather complicated
and call a lot of other programs and bc was the only one that has this issue,
so, despite no specification defines who should clear process signal mask,
caller or callee, I think this should be fixed with simple patch attached.

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