add hijack option on mount to allow replacment. not enabled by default

This allows an incoming source client to replace an existing stream, typical use
would be to replace a relay which restarts again on exit.  This would happen
before listeners drop via an fallback.

src/auth.c

@@ -939,6 +939,8 @@ int auth_check_source (client_t *client, const char *mount)
         if (mountinfo)
         {
             ret = 1;
+            if (mountinfo->hijack)
+                client->flags |= CLIENT_HIJACKER;
             if (auth_stream_authenticate (client, mount, mountinfo) > 0)
                 break;
             ret = -1;

src/cfgfile.c

@@ -1404,6 +1404,7 @@ static int _parse_mount (cfg_xml *cfg, void *arg)
         { "fallback-mount",     _parse_fallback,    &mount->fallback },
         { "fallback-override",  config_get_bool,    &mount->fallback_override },
         { "fallback-when-full", config_get_bool,    &mount->fallback_when_full },
+        { "hijack",             config_get_bool,    &mount->hijack },
         { "allow-chunked",      config_get_bool,    &mount->allow_chunked },
         { "max-listeners",      config_get_int,     &mount->max_listeners },
         { "max-bandwidth",      config_get_bitrate, &mount->max_bandwidth },

src/cfgfile.h

@@ -169,6 +169,7 @@ typedef struct _mount_proxy {
     int allow_chunked; /* allow chunked transfers */
     int mp3_meta_interval; /* outgoing per-stream metadata interval */
     int max_send_size;
+    int hijack;                 // an authenticated source can hijack and exist stream
     int filter_theora; /* prevent theora pages getting queued */
     int url_ogg_meta; /* enable to allow updates via url requests for ogg */
     int ogg_passthrough; /* enable to prevent the ogg stream being rebuilt */