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