- Backout last change, since it is memory overkill for a non busy host or

- Backout last change, since it is memory overkill for a non busy host or
  for a notebook with em(4) adapter.
- Introduce tunables em.hw.txd and em.hw.rxd, which allow administrator
  to configure number of transmit and receive descriptors.
- Check em.hw.txd and em.hw.rxd against hardware limits [*] and require
  them to be multiple of 128.

[*] According to comments in if_em.h the 82540EM/82541ER chips can handle
    more than 256 descriptors. Since we don't have this hardware to test,
    we decided to mimic NetBSD wm(4) driver, that limits these chips to
    256 descriptors.

In collaboration with:	yongari