Remove unnecessary apr_table_do() function casts

Function casts can cause hard-to-debug corruption issues if a
declaration is accidentally changed to be incompatible. Luckily, most of
the function casts for apr_table_do() calls are unnecessary. Remove
them, and adjust the signatures for helpers that weren't taking void* as
the first argument.

The remaining helper that requires a cast is http_filter.c's
form_header_field(), which is probably where many of these casts were
copy-pasted from. I have left it as-is: it has other direct callers
besides apr_table_do(), and it's already documented with warnings not to
change the function signature.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769192 13f79535-47bb-0310-9956-ffa450edef68