Remove unused count from pseudo-device md.

Changes to MEMORY_RBFLAGS option:
- renamed to MEMORY_DISK_RBFLAGS to better fit the rest of the
MEMORY_DISK options(4)
- change default value to RB_AUTOBOOT instead of RB_SINGLE, and adapt
the confi

Changes to MEMORY_RBFLAGS option:
- renamed to MEMORY_DISK_RBFLAGS to better fit the rest of the
MEMORY_DISK options(4)
- change default value to RB_AUTOBOOT instead of RB_SINGLE, and adapt
the config(5) files accordingly
- document this option inside options(4)

See also http://mail-index.netbsd.org/tech-kern/2008/12/25/msg003924.html

Reviewed by abs@ in private mail.

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Rename MEMORY_DISK_SIZE (formerly MINIROOTSIZE) to MEMORY_DISK_ROOT_SIZE,
which was suggested by Izumi Tsutsui <tsutsui@ceres.dti.ne.jp> as
being more consistent with what it's controlling...

* rename MINIROOTSIZE to MEMORY_DISK_SIZE, so that all md(4) options
are now consistently named
* fold opt_mdsize.h into opt_md.h

downsized kernels for installation & recovery