document 'devs' and 'procs' commands.

add a 'devs' script to display a list of all devices by device_t,
name, parent, private data.

for procs, put the printf \n into the same line. non trivial
speed up with large process table.

Add the lwp address so you don't have to leave gdb and use ps to grab an lwp's
stack.

- Replace 'xps' with 'procs' and 'lwps'.
- Needs a bit more work.