Clarify --no-sendfile default

The --no-sendfile option had a confusing entry in the usage message.
Even though sendfile is enabled by default, the --no-sendfile flag
showed a true value as the default, which could be interpreted to
mean that by default sendfile support is disabled.

This change makes the default "None", meaning sendfile is not
disabled, which is hopefully slightly more clear.

Close #1156

gunicorn/config.py

@@ -821,13 +821,14 @@ class PreloadApp(Setting):
         restarting workers.
         """
 
+
 class Sendfile(Setting):
     name = "sendfile"
     section = "Server Mechanics"
     cli = ["--no-sendfile"]
     validator = validate_bool
-    action = "store_false"
-    default = True
+    action = "store_const"
+    const = False
     desc = """\
         Disables the use of ``sendfile()``.
 
@@ -837,6 +838,7 @@ class Sendfile(Setting):
            disabling.
         """
 
+
 class Chdir(Setting):
     name = "chdir"
     section = "Server Mechanics"

gunicorn/http/wsgi.py

@@ -347,7 +347,7 @@ def write(self, arg):
         util.write(self.sock, arg, self.chunked)
 
     def can_sendfile(self):
-        return self.cfg.sendfile and sendfile is not None
+        return self.cfg.sendfile is not False and sendfile is not None
 
     def sendfile(self, respiter):
         if self.cfg.is_ssl or not self.can_sendfile():