Make the argument to checked_fd_isset non-const.

I got this error compiling on OS X 10.6:
In file included from netutil.cc:132:
../nbase/nbase.h: In function 'int checked_fd_isset(int, const fd_set*)':
../nbase/nbase.h:385: error: invalid conversion from 'const fd_set*' to 'fd_set*'
../nbase/nbase.h:385: error:   initializing argument 2 of 'int __darwin_fd_isset(int, fd_set*)'
netutil.cc: In function 'int send_ipv6_ip(const sockaddr_in6*, const unsigned char*, size_t)':
netutil.cc:3846: warning: unused variable 'tclass'
make[2]: *** [netutil.o] Error 1
make[1]: *** [netutil_build] Error 2