Use FORMAT everywhere, and fix up the errors it finds

Use FORMAT everywhere, and fix up the errors it finds

Many printf-like variadic functions weren't properly decorated with
FORMAT annotations.  Add them everywhere.

It is not possible to annotate a function pointer type, so that
doesn't work for the function pointers in struct log_if.  However, we
have convenience wrapper functions slilog and vslilog, which are
appropriately decorated and therefore safer.  Change all call sites to
use those instead, and leave a comment.  (Rename the function pointer
variable names so that we don't miss any call sites.)

Fix the bugs that this new compiler checking reveals.  These are
nearly all in fatal error reporting, and not very scary.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>