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.
Then this is wrong too: https://github.com/manuelm/libelektra/blob/clang/tests/cframework/tests.h#L152 ?