Add :column info for several linters, and other small improvements

For the following linters, added :column info, and removed namespace
from vars reported, since they should always be in the namespace being
linted:

+ misplaced-docstrings
+ redefd-vars
+ def-in-def

Also made line number info for redefd-vars better (i.e., less often
nil), although still not in all cases.

For the following linters, added :column info only, but kept namespace
in vars reported, since they are often in a different namespace than
where the warning occurs:

+ wrong-arity

Added 2 simple test cases for wrong-arity linter.