Add format attribute and fix issues

Add format attribute and fix issues

Add __attribute__((format)) to log_filep() and _show() functions, fixing
the bugs they found in the source code.

For functions that receive va_list instead of being variadic functions
we put 0 in the last argument, so at least the string is checked and we
get warnings of -Wformat-nonliteral type. So, it's better than adding a
pragma here to shut up the warning.