Allow wildcard aliases in --bridge-interface option

This is useful when using dnsmasq as DHCP server for a set of VMs
whose data is routed by the host instead of being bridged.  In this
scenario:

- There is an unbounded set of TAP interfaces that have no IP address
  at the host end.

- DHCP allocation is done from an IPv4 address range associated with a
  dummy interface.

- We run dnsmasq with --interface dummy --interface tap*
  --bind-dynamic, so that it listens on all the TAP interfaces, and
  --bridge-interface=dummy,tap*, so that it will allocate IP addresses
  via the TAP interfaces from the range associated with the dummy
  interface.

src/dhcp.c

@@ -232,7 +232,7 @@ void dhcp_packet(time_t now, int pxe_fd)
   for (bridge = daemon->bridges; bridge; bridge = bridge->next)
     {
       for (alias = bridge->alias; alias; alias = alias->next)
-	if (strncmp(ifr.ifr_name, alias->iface, IF_NAMESIZE) == 0)
+	if (wildcard_matchn(alias->iface, ifr.ifr_name, IF_NAMESIZE))
 	  {
 	    if (!(iface_index = if_nametoindex(bridge->iface)))
 	      {

src/dnsmasq.h

@@ -1150,6 +1150,7 @@ void bump_maxfd(int fd, int *max);
 int read_write(int fd, unsigned char *packet, int size, int rw);
 
 int wildcard_match(const char* wildcard, const char* match);
+int wildcard_matchn(const char* wildcard, const char* match, int num);
 
 /* log.c */
 void die(char *message, char *arg1, int exit_code);

src/util.c

@@ -602,3 +602,22 @@ int wildcard_match(const char* wildcard, const char* match)
 
   return *wildcard == *match;
 }
+
+/* The same but comparing a maximum of NUM characters, like strncmp.  */
+int wildcard_matchn(const char* wildcard, const char* match, int num)
+{
+  while (*wildcard && *match && num)
+    {
+      if (*wildcard == '*')
+        return 1;
+
+      if (*wildcard != *match)
+        return 0; 
+
+      ++wildcard;
+      ++match;
+      --num;
+    }
+
+  return (!num) || (*wildcard == *match);
+}