init.sh: penalize a set-x-impaired shell; don't disqualify it

init.sh: penalize a set-x-impaired shell; don't disqualify it

* tests/init.sh: Too many shells corrupt application stderr when
you set -x, so we can't afford to disqualify them, since at least
on Irix-6.5, that would disqualify all bourne shells.
Instead, use a more discerning approach.
When iterating through candidate shells, give each a score
of 10 for perfect, 9 if corrupts stderr upon set -x but passes
all other tests, and some other value if it is unacceptable.
Use the first shell that gets a score of 10.
If no shell scores 10, use the first that scored 9.
Finally, when VERBOSE=yes is requested and set -x might cause trouble, simply
issue a warning and refrain from enabling debug output.