ETHER: Add support for generated interface MAC addresses

sim> SET XQ MAC=aa:bb:cc:dd:ee:ff{/bits}{>filespec}

where:

- all of the aa:bb:cc:dd:ee:ff values must be hex digits
- bits is the number of bits which are to be taken from the supplied
   MAC aa:bb:cc:dd:ee:ff with legal values from 16 to 48 and a default
   of 48 bits.
- filespec specifies a file which contains the MAC address to be used
   and if it doesn't exist an appropriate generated address will be stored
   in this file and a subsequent SET MAC invocation specifying the same
   file will use the value stored in the file rather than generating a new
   MAC.

As discussed in #317

sim_ether.h

@@ -359,6 +359,8 @@ void eth_show_dev (FILE*st, ETH_DEV* dev);              /* show ethernet device
 
 void eth_mac_fmt (ETH_MAC* const add, char* buffer);    /* format ethernet mac address */
 t_stat eth_mac_scan (ETH_MAC* mac, const char* strmac); /* scan string for mac, put in mac */
+t_stat eth_mac_scan_ex (ETH_MAC* mac,                   /* scan string for mac, put in mac */
+                        const char* strmac, UNIT *uptr);/* for specified unit */
 
 t_stat ethq_init (ETH_QUE* que, int max);               /* initialize FIFO queue */
 void ethq_clear  (ETH_QUE* que);                        /* clear FIFO queue */