Sync several manpages in section 9 with reality and clean up a bit.

Sync several manpages in section 9 with reality and clean up a bit.

DEV_MODULE(9):
    - adjust example

devtoname(9):
    - dev_t is cdev_t now

make_dev(9):
    - dev_t is cdev_t now
    - make_dev() takes dev_ops as its first argument
    - obtain a reference to cdev_t, so we can destroy it later
    - don't forget to call dev_ops_remove() during shutdown

vcount(9):

    - document count_udev(9)

Submitted-by:  Stathis Kamperis <ekamperi@gmail.com>
Dragonfly-bug: <http://bugs.dragonflybsd.org/issue1269>