scripts: improve rendering of synopsis/usage

Subcommands "--help" is now rendered as:

```
 tahoe [global-options] COMMAND [options] ARGS
 (use 'tahoe --help' to view global options)
 USAGE (flags/options)
 DESCRIPTION
 DESCRIPTION_UNWRAPPED
```

The new .description and .description_unwrapped fields allow
commands (subclasses of twisted.python.usage.Usage) better control over
how their explanations are rendered: the old .longdesc field was wrapped
unpleasantly.

src/allmydata/scripts/runner.py

@@ -66,11 +66,15 @@ class Options(usage.Options):
         print >>self.stdout, allmydata.get_package_versions_string(show_paths=True, debug=True)
         self.no_command_needed = True
 
-    def getSynopsis(self):
-        return "\nUsage: tahoe [global-opts] <command> [command-options]"
+    def __str__(self):
+        return ("\nUsage: tahoe [global-options] <command> [command-options]\n"
+                + self.getUsage())
+
+    synopsis = "\nUsage: tahoe [global-opts]" # used only for subcommands
 
     def getUsage(self, **kwargs):
         t = usage.Options.getUsage(self, **kwargs)
+        t = t.replace("Options:", "\nGlobal options:", 1)
         return t + "\nPlease run 'tahoe <command> --help' for more details on each command.\n"
 
     def postOptions(self):