tap/awk: support colorized output

tap/awk: support colorized output

* lib/tap-driver.sh: Pass definition of colors to the BEGIN block
of the awk script, using ...
($init_colors): ... this new variable, which supersedes ...
($red, $grn, $lgn, $blu, $mgn, $std): ... these variables, which
have been consequently removed.
($color_tests): We don't have to pass this anymore to the awk
script, so do not convert it to a numeric boolean.
(decorate_result): Implement for real.

With this change, the test 'tap-color.test' now passes.