Add a few examples
Print an error message on kernel crashes
Add a --watchdog flag so qemu can detect crashes

Not enabled by default because it requires a kernel
module and conflicts with gdb.
Fix UML GDB

F1a593260ac98cf28688ac0d96de50f4a4b96441 broke it.
Cleaner PWD handling

Don't convert $PWD to cwd in case they don't match; do pass PWD
and let the shell use it or override it (see pwd -L).