test: udp-preload: Beginning of actual implementation

test: udp-preload: Beginning of actual implementation

  gcc -Wall -D_REENTRANT -fPIC -c udp-preload.c && \
  ld -shared -soname foo.so.1 udp-preload.o -o udp-preload.so -ldl -lc

produces a library with which

  LD_PRELOAD=test/udp-preload.so test/invoke

produces various complaints like

  udp (test/tmp/outside.conf:19): setsockopt(,IPV6_V6ONLY,&1,): Operation not supported
  udp (test/tmp/inside.conf:19): socket [::]:16913 experiencing some trouble transmitting IPv6 (to [::1]:16900): Bad file descriptor

This is progress.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>