tap: scripts with a SKIP plan but with exit status != 0 must error

tap: scripts with a SKIP plan but with exit status != 0 must error

This change has been motivated by Automake's own testsuite.  Some
TAP tests there were erroring out (due to unexpected, unhandled
failures) before having encountered TAP result, so that the
simple-minded shell library implementing TAP generation ended up
printing a "1..0" trailing test plan; this caused the script to be
reported as a SKIP rather than an ERROR -- a nasty false negative.

* lib/tap-driver: Add prototypes for each subroutine, to free up
the order in which they can be defined and called.
(main): Move the code checking for a bad exit status of the TAP
producer ...
(finish): ... here, and flush the TAP stream to ensure that the
parser always obtains the producer's exit status.
* tests/tap-skip-whole-badexit.test: New test.
* tests/Makefile.am (tap_with_common_setup_tests): Add it.