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.