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.
This also fixes a bug in "e6996c5 rmmod: route all messages to syslog if
told to" in which "+ verbose" was removed. Instead of letting verbose
add to kmod_get_log_priority(), let it be similar to the other programs
instead.
