[YoutubeDL] Rename alternative proxy to China verification proxy

Documentations are updated as well to make the purpose of the new
option more clear

youtube_dl/YoutubeDL.py

@@ -1721,7 +1721,7 @@ class YoutubeDL(object):
         self._write_string('[debug] exe versions: %s\n' % exe_str)
 
         self.dump_proxy_map('default', 'Proxy map')
-        self.dump_proxy_map('alternative', 'Alternative proxy map')
+        self.dump_proxy_map('cn_verification', 'China IP verification proxy map')
 
         if self.params.get('call_home', False):
             ipaddr = self.urlopen('https://yt-dl.org/ip').read().decode('utf-8')
@@ -1761,12 +1761,13 @@ class YoutubeDL(object):
         ydlh = YoutubeDLHandler(self.params, debuglevel=debuglevel)
 
         default_proxy = self.params.get('proxy')
-        alternative_proxy = self.params.get('alternative_proxy')
-        if not alternative_proxy:
-            alternative_proxy = default_proxy
+        cn_verification_proxy = self.params.get('cn_verification_proxy')
+
+        if not cn_verification_proxy:
+            cn_verification_proxy = default_proxy
 
         self._setup_single_opener('default', default_proxy, https_handler, ydlh)
-        self._setup_single_opener('alternative', alternative_proxy, https_handler, ydlh)
+        self._setup_single_opener('cn_verification', cn_verification_proxy, https_handler, ydlh)
 
     def _setup_single_opener(self, opener_name, opts_proxy, https_handler, ydlh):
         cookie_processor = compat_urllib_request.HTTPCookieProcessor(

youtube_dl/extractor/letv.py

@@ -57,7 +57,7 @@ class LetvIE(InfoExtractor):
             'publish time'
         ],
         'params': {
-            'alternative_proxy': 'proxy.uku.im:8888'
+            'cn_verification_proxy': 'proxy.uku.im:8888'
         }
     }]
 
@@ -94,7 +94,7 @@ class LetvIE(InfoExtractor):
 
         play_json = self._download_json(
             'http://api.letv.com/mms/out/video/playJson?' + compat_urllib_parse.urlencode(params),
-            media_id, 'playJson data', opener_name='alternative')
+            media_id, 'playJson data', opener_name='cn_verification')
 
         # Check for errors
         playstatus = play_json['playstatus']

youtube_dl/options.py

@@ -177,9 +177,9 @@ def parseOpts(overrideArguments=None):
         default=None, metavar='URL',
         help='Use the specified HTTP/HTTPS proxy. Pass in an empty string (--proxy "") for direct connection')
     network.add_option(
-        '--alternative-proxy', dest='alternative_proxy',
+        '--cn-verification-proxy', dest='cn_verification_proxy',
         default=None, metavar='URL',
-        help='Use the specified HTTP/HTTPS address as the alternative proxy. Pass in an empty string (--alternative-proxy "") for direct connection'
+        help='The proxy used in the China IP verification step. The default proxy specified by --proxy is used if this option is not given'
     )
     network.add_option(
         '--socket-timeout',