Squashed commit of the following:

Author: Gregory Nutt <gnutt@nuttx.org>

    Run all .h and .c files modified in last PR through nxstyle.

Author: Xiang Xiao <xiaoxiang@xiaomi.com>

    Net cleanup (#17)

    * Fix the semaphore usage issue found in tcp/udp

    1. The count semaphore need disable priority inheritance
    2. Loop again if net_lockedwait return -EINTR
    3. Call nxsem_trywait to avoid the race condition
    4. Call nxsem_post instead of sem_post

    * Put the work notifier into free list to avoid the heap fragment in the long run.  Since the allocation strategy is encapsulated internally, we can even refine the implementation later.

    * Network stack shouldn't allocate memory in the poll implementation to avoid the heap fragment in the long run, other modification include:

    1. Select MM_IOB automatically since ICMP[v6] socket can't work without the read ahead buffer
    2. Remove the net lock since xxx_callback_free already do the same thing
    3. TCP/UDP poll should work even the read ahead buffer isn't enabled at all

    * Add NET_ prefix for UDP_NOTIFIER and TCP_NOTIFIER option to align with other UDP/TCP option convention

    * Remove the unused _SF_[IDLE|ACCEPT|SEND|RECV|MASK] flags since there are code to set/clear these flags, but nobody check them.

net/udp/udp_callback.c

@@ -234,7 +234,7 @@ static uint16_t udp_datahandler(FAR struct net_driver_s *dev,
       return 0;
     }
 
-#ifdef CONFIG_UDP_NOTIFIER
+#ifdef CONFIG_NET_UDP_NOTIFIER
   /* Provided notification(s) that additional UDP read-ahead data is
    * available.
    */