cwchar:Use CONFIG_LIBC_WCHAR to only export the wc/mb functions

Approved-by: Gregory Nutt <gnutt@nuttx.org>

   When a build does not want to use wide or multibyte char
   CONFIG_LIBC_WCHAR is not set. Therefore we should to only
   export the wc/mb functions when defined.

   Regardless of the stat of CONFIG_LIBC_WCHAR the non mb/wc
   definitions such as mbstate_t, wint_t, wctype_t need to be
   exported.

Network routing table:  Trivial name changes to make IPv4 and IPv6 more symmetric.  Add debug logic to dump the content of the router table.

Networking: Fix typo for 802.11 devices in netdev_register().  Was being masked before because depended on CONFIG_NET_MULTLINK.

Fix IPv6 loopback driver that depended on the removed g_ipv6_allonesaddr.  Noted by Masayuki Ishikawa

readme: fix typo Bask->Bash

Approved-by: Gregory Nutt <gnutt@nuttx.org>

Networking:  Fix netdev_dev_lladdrsize.  In some configurations, it could return the wrong size for the address of a packet radio.

Remove CONFIG_NET_MULTILINK.  This increases code size by a little, but greatly reduces the complexity of the network code.

Remove CONFIG_NETDEV_MULTINIC.  This increases code size by a little, but greatly reduces the complexity of the network code.

libm: Eliminate a warning from lgamma();  Update some comments; Add some logic to handle broadcast better -- Currently conditioned out because it does not work.

ICMPv6: Support source link-layer address option in RA

Approved-by: Gregory Nutt <gnutt@nuttx.org>

C Coding Standard:  Clarify the form of structures and unions declared with local variable definition.

IPv6:  Remove comparisons to the address with all ones set.  IPv6 does not support broadcast addresses and certainly not in that form.  Replace with multicast addresses beginning with 0xff02.

I discovered while working on the SAMV7 mcan driver that the implementation of the CAN error handling is suboptimal. In the current implementation the following errors are implemented as pending errors:

* Receiving
  * MCAN_INT_STE (Stuff Error)
    More than 5 equal bits in a sequence occurred.
  * MCAN_INT_CRCE (CRC Error)
    Received CRC did not match the calculated CRC.
  * MCAN_INT_RF0L (Receive FIFO 0 Message Lost)
    Receive FIFO 0 message lost, also set after write attempt to Receive FIFO 0 of size zero.
  * MCAN_INT_RF1L (Receive FIFO 1 Message Lost)
    Receive FIFO 1 message lost, also set after write attempt to Receive FIFO 1 of size zero.

* Sending
  * MCAN_INT_BE (Bit Error)
    Device wanted to send a rec / dom level, but monitored bus level was dominant / recessive.
  * MCAN_INT_TEFL (Tx Event FIFO Element Lost)
    Tx Event FIFO element lost, also set after write attempt to Tx Event FIFO of size zero.

* General
  * MCAN_INT_MRAF (Message RAM Access Failure)
    The flag is set, when the Rx Handler
    * has not completed acceptance filtering or storage of an accepted message until the arbitration field of the following message has been received. In this case acceptance filtering or message storage is aborted and the Rx Handler starts processing of the following message.
    * was not able to write a message to the Message RAM. In this case message storage is aborted. In both cases the FIFO put index is not updated resp. the New Data flag for a dedicated Receive Buffer is not set, a partly stored message is overwritten when the next message is stored to this location. The flag is also set when the Tx Handler was not able to read a message from the Message RAM in time. In this case message transmission is aborted. In case of a Tx Handler access failure the MCAN is switched into Restricted Operation mode (see Section 47.5.1.5). To leave Restricted Operation mode, the processor has to reset MCAN_CCCR.ASM.
  * MCAN_INT_ELO (Error Logging Overflow)
    Overflow of CAN Error Logging Counter occurred.

The listed errors are not pending, the errors occurred and are gone directly afterwards. This commit changes the described behavior and simplifies the handling of CAN errors.