This replaces the previous allocation bitmask, which only supported
small channel-sets, with the first-fit allocator. Minors are always
taken from the smallest available slot starting from minor 0.

Each cset is allocated separately, but this should not be a problem.

With the zio-mini driver, which is not upstream yet, this works:

     insmod zio-mini.ko  nchan=300 ndev=20

And we get all the devices we need.

    # ls -1 /sys/dev/char/250:* | wc -l
    12000

The patch makes also some other renaming of functions (less underscores
for functions exported by zio-cdev.c to zio-sys.c) and similar stuff
that makes stuff clearer, if I happened to find them during this change.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>