[roll] Roll fuchsia [debugger] Unify embedded mode transition on process exit

Currently when running zxdb in "embedded" mode with another program
streaming output through a named piped via the --stream-fd option, it
can be quite easy to type the wrong command and end up in a state where
the zxdb prompt is still active and showing debugger output, and the
parent process's output is not seen from the named pipe, or even worse,
zxdb exits and the there is no output at all even though the parent
process is still happily working.

With this change, the behavior of the following commands are changed in
embedded mode:

"quit": Will not exit the debugger nor terminate the embedded fd
streamer. Instead, all currently attached processes will be detached and
the console will unconditionally return to embedded mode.

"ctrl-d": Will not exit the debugger, the currently attached process
will be detached. If no other processes are currently attached then the
console will return to embedded mode, otherwise the debugging session is
considered to remain interactive.

"detach": Explicitly detaching from a process will return the debugger
to embedded mode if there are no other attached processes.

"continue": Continue will never return the console to embedded mode. If
the process exits as a result of the continue, the console will
automatically return to embedded mode if there are no other attached
processes.

Other, manual commands, like "process X rm" will also trigger the
console to transition to embedded mode.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/995452
Original-Revision: 18f041739c28b9b7ddeb3d8d3e22281c476731aa
GitOrigin-RevId: 74dfa70d8ef068d9f1e59e2b6739da13b4af229f
Change-Id: I87259a94c408ab4e06fafa3d1706672d7c0619f1