Merge branch 'jk/gcc-function-attributes'

Use the function attributes extension to catch mistakes in use of our own variadic functions that use NULL sentinel at the end (i.e. like execl(3)) and format strings (i.e. like printf(3)). * jk/gcc-function-attributes: Add the LAST_ARG_MUST_BE_NULL macro wt-status: use "format" function attribute for status_printf use "sentinel" function attribute for variadic lists add missing "format" function attributes