Importing usr.bin/write

Replaces commands/write. No Minix-specific changes needed.

The NetBSD version lacks a few features that were present
in the Minix version: cbreak mode, verbose, and shell escapes,
but the main write(1) functionality is there and working.

Change-Id: I87b9589c54d3595d26247d221bb3d1f613feeb8c