configure: consistent command -v usage

When the shell builtin `command -v` operates normally, it either
prints the path of the arg given to it and returns zero -- or it
returns nonzero and prints nothing.
In abnormal situations something might be printed to stderr and
in that case we want to know about it; therefore the used
command -v stderr redirections to /dev/null have been removed.

The `hash` (builtin) command in ksh returns zero even the arg
given to is is not found in path. For that and for consistency
the one appearance of it has been converted to `command -v`.

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