tap/perl: don't redirect perl warnings/errors to log files

tap/perl: don't redirect perl warnings/errors to log files

With this change, the test `tap-driver-stderr.test' also passes
with the perl implementation of the TAP driver.

* lib/tap-driver.pl (start): Save the original stderr into the
`OLDERR' file handle, and call ...
(trap_perl_warnings_and_errors): ... this new function, trapping
the `__WARN__' and `__DIE__' "pseudo-signals" to ensure that the
warning and error messages goes to that original stderr stream.
Since we are at it, be sure to prepend all possible "die" message
with the name of the script as given by the `$ME' variable.