dnsmasq: skip options that are not compiled in

This is to make life easier for users with customized build of
dnsmasq-full variant.  Currently dnsmasq config generated by current
service script will be rejected by dnsmasq build lacking DHCP feature

 - Options like --dhcp-leasefile have default values.  Deleting them
   from uci config or setting them to empty value will make them take on
   default value in the end
 - Options like --dhcp-broadcast are output unconditionally

Tackle this by

 - Check availablility of features from output of "dnsmasq --version"
 - Make a list of options guarded by HAVE_xx macros in src/options.c of
   dnsmasq source code
 - Ignore these options in xappend()

Two things to note in this implementation

 - The option list is not exhaustive.  Supposedly only those options that
   may cause dnsmasq to reject with "unsupported option (check that
   dnsmasq was compiled with DHCP/TFTP/DNSSEC/DBus support)" are taken
   into account here
 - This provides a way out but users' cooperation is still needed.  E.g.
   option dnssec needs to be turned off, otherwise the service script
   will try to add --conf-file pointing to dnssec specific anchor file
   which dnsmasq lacking dnssec support will reject

Resolves FS#2281

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

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