tap: add a dummy TAP driver script implemented in shell + awk

tap: add a dummy TAP driver script implemented in shell + awk

The user can also now decide which implementation of the TAP driver
to use in the testsuite by defining the `$am_tap_implementation'
variable to either "perl" or "shell".  Future enhancements will
allow the testsuite to automatically run the test scripts on TAP
support with both the TAP driver implementations, to improve
coverage.

* tests/defs (fetch_tap_driver): Honor the `$am_tap_implementation'
variable to decide which implementation of the TAP driver to fetch.
($am_tap_implementation): Default to "perl".
* tests/tap-common-setup.test: Do not fetch the TAP driver, the
code in tap-setup.sh does that already (and respecting runtime
overriding of `$am_tap_implementation').
* lib/tap-driver: Renamed ...
* lib/tap-driver.pl: ... to this, and ...
($ME): ... adjusted this.
* doc/automake.texi: Adjust to the renaming.
* tests/Makefile.am (TAP_LOG_DRIVER): Likewise.
* tests/tap-doc2.test: Likewise.
* lib/tap-driver.sh: New script, still mostly dummy.
* lib/Makefile.am (dist_script_DATA): Update, and since we are at
it, rewrite it to make it easier to add new entries in the future.