tap: real (but still incomplete) awk implementation of TAP driver

tap: real (but still incomplete) awk implementation of TAP driver

* lib/tap-driver.sh : Add an incomplete, but mostly working,
implementation of a TAP parser and driver in awk.  It doesn't yet
support colorized output, fetching of exit status from test
programs, nor a way to escape TAP directives in TAP result lines,
but passes all the tests of TAP support in the automake testsuite,
apart from the following ones:
  - tap-color.test
  - tap-escape-directive.test
  - tap-exit.test
  - tap-missing-plan-and-bad-exit.test
  - tap-passthrough-exit.test
  - tap-planskip-badexit.test
  - tap-planskip-unplanned-corner.test
  - tap-signal.test

Tested on Debian GNU/Linux with GNU awk 3.1.7 and 3.0.2, "original
awk" 2010-05-23-1, and mawk 1.3.3-15 (with which also the test
`tap-realtime.test' fails, in addition to those listed above).