work in progress: screen_api_test

get rid of api_test and have screen_api_test write all of
its test results to a file. require modified ccan/tap module
fix linking of shared tap object & api_test.h bug

i think that tap.so has to come before libccan.a so that tap
symbols resolve from tap.so and not libccan.a

in api_test.h we were closing the tap output file in the child
but still using tap diag function calls. somehow this messes up the
child such that none of the data written to the pipe makes it through
to the parent.
add valgrind version of screen_api_test

also take screen_api_test out of the "tests" target.

also checkout "cantora" branch of ccan.
cleanup some dead code