Bring in some iostat fixes that bde reminded me about. These fixes were
originally written in January, 2000, but have been substantially updated.

• No longer use hz/stathz and the CPU times in computing the TTY stats, but rather use etime, like the disk stats.
• Clean up malloc/realloc failure tests.
• Use a new integrated routine to fetch devstat information via sysctl or KVM.
• Get rid of the X() macro for calculating CPU stats
• Use rint() on the CPU state display to avoid truncation errors. (this requires libm)
• Clean up flag usage somewhat.

Reviewed by: bde