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.
I think using both `keyNew(NULL)` and `keyNew(0)` is inconsistent with the use of a key constructor with arguments (e.g. `keyNew("foo", KEY_END)`). Nonetheless I fully agree that using this is preferrable to producing a compiler warning. I just wanted to point out that we should not give up the search / hope for a better solution as the this solution is still not optimal. But for now I am happy that the warning is gone :).