Added a config directive for a Unix socket mask

Added a configuration directive to allow a user to specify the
permissions to be granted to the Unix socket file.  I followed
the format Pieter and Salvatore discusses in issue #85 (
https://github.com/antirez/redis/issues/85).

src/redis.h

@@ -378,6 +378,7 @@ struct redisServer {
     int port;
     char *bindaddr;
     char *unixsocket;
+    mode_t unixsocketperm;
     int ipfd;
     int sofd;
     redisDb *db;