tools/fw: Fix set-but-unused gcc47 warning in generated C file

Only generate parent and the assignment code, if there are more
then 1 input firmware files.
mxge: Fix set-but-unused gcc47 warning
tip(1): Fix some -Wmissing-parameter-type warnings.

It still needs more cleanup.
Revert "Revert "i386/files: isa/clock.c is clean enough to compile w/ -Werror""

This reverts commit 9b40fbdac1da8214991bb70ab4767862203f2375.

Now that we no longer have gcc41, it compiles without warnings on both of
our compilers.
kernel/virtio: Remove some unused variables.
kernel/virtio: Remove some unneeded inclusions of <sys/cdefs.h>.
kernel/virtio: Constify some struct members to silence warnings.
kernel/firewire: Remove #ifdef __DragonFly__ jungle.