Add --dhcp-option-pxe config.

This acts almost exactly like --dhcp-option except that the defined option
is only sent when replying to PXE clients. More importantly, these
options are sent in reply PXE clients when dnsmasq in acting in PXE
proxy mode. In PXE proxy mode, the set of options sent is defined by
the PXE standard and the normal set of options is not sent. This config
allows arbitrary options in PXE-proxy replies. A typical use-case is
to send option 175 to iPXE. Thanks to Jason Berry for finding the
requirement for this.

src/rfc3315.c

@@ -1317,7 +1317,7 @@ static struct dhcp_netid *add_options(struct state *state, int do_refresh)
 {
   void *oro;
   /* filter options based on tags, those we want get DHOPT_TAGOK bit set */
-  struct dhcp_netid *tagif = option_filter(state->tags, state->context_tags, daemon->dhcp_opts6);
+  struct dhcp_netid *tagif = option_filter(state->tags, state->context_tags, daemon->dhcp_opts6, 0);
   struct dhcp_opt *opt_cfg;
   int done_dns = 0, done_refresh = !do_refresh, do_encap = 0;
   int i, o, o1;