Add a new question "Escape to shell before reboot? [no]" at the end of

Add a new question "Escape to shell before reboot? [no]" at the end of
the install and upgrade process.

"no" or [enter] reboots the system
"yes" leaves the install or upgrade at the shell prompt

On selected architectures, sparc64 being the only so far, the system
is halted instead of rebooted.

autoinstall(8) is NOT affected by this new question as it reboots the
system automatically already.

Suggested by and OK deraadt@, tb@ phessler@
Feedback on sparc64 by stsp@
Positive feedback pea@ landry@