pyprog: Wrap entry-point call with sys.exit()

Reading this spec, we probably want to change
```
  entry_run = 'from %s import %s; %s()' % (mod, func, func)
```
to
```
  entry_run = 'import sys; from %s import %s; sys.exit(%s())' % (mod, func, func)
```
to follow
https://packaging.python.org/en/latest/specifications/entry-points/#use-for-scripts
, this is also what `zc.recipe.egg` does.

/reviewed-by @kirr
/reviewed-on !1108

component/pygolang/buildout.cfg

@@ -77,9 +77,9 @@ init =
 
   options['depends'] += '$${.%s.pyprog:recipe}' % name
 
-  # mod:func -> 'from mod import func; func()'
+  # mod:func -> 'from mod import func; sys.exit(func())'
   mod, func = entry.split(':')
-  entry_run = 'from %s import %s; %s()' % (mod, func, func)
+  entry_run = 'from %s import %s; sys.exit(%s())' % (mod, func, func)
 
   # indent pyinit with '  '
   __pyinit = '\n'.join(['  '+_ for _ in pyinit.splitlines()])
@@ -95,6 +95,7 @@ init =
   template=
     inline:
     #!$${.%(name)s.pyexe:exe}
+    import sys
   %(__pyinit)s
     %(entry_run)s