Fix clang warnings

This should fix most of the clang warnings for default plugins.

One warning I wasn't able to fix is:
./src/include/kdberrors.h:41:42: warning: token pasting of ',' and \_\_VA_ARGS\_\_ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]

There's a discussion about a standard alternative to \_\_VA_ARGS\_\_ at http://stackoverflow.com/questions/5588855/standard-alternative-to-gccs-va-args-trick but the argument counting seems awkward.

I'll look into the nodep-plugins now.
You wrote:
> So what to do? clang complains that we can't cast a enum (KEY_END) to a
> char pointer. Imho this is a valid complain. Shall I replace this with "0,
> KEY_END" instead?

Nothing is to do, because it is unclear how the sentinel for enums, if it will 
ever exist, would work. And keyNew(0) is much shorter.