Make test(1) complain when we pass unexpected extra args
instead of silently ignoring them or returning TRUE.
This is a bit more consistent with PoSix systems and
avoids confusion (fgb thought that test -dw /tmp would
check if /tmp was a writable dir).

Also complain when the argument to -t is not a valid
filedes.

Fix small typo in the man page and document that there
is on way to test if a dir is writeable.

uriel