monitor: Separate "default monitor" and "current monitor" cleanly

Commits 376253ec..731b0364 introduced global variable cur_mon, which
points to the "default monitor" (if any), except during execution of
monitor_read() or monitor_control_read() it points to the monitor from
which we're reading instead (the "current monitor").  Monitor command
handlers run within monitor_read() or monitor_control_read().

Default monitor and current monitor are really separate things, and
squashing them together is confusing and error-prone.

For instance, usb_host_scan() can run both in "info usbhost" and
periodically via usb_host_auto_check().  It prints to cur_mon, which
is what we want in the former case: the monitor executing "info
usbhost".  But since that's the default monitor in the latter case, it
periodically spams the default monitor there.

A few places use cur_mon to log stuff to the default monitor.  If we
ever log something while cur_mon points to current monitor instead of
default monitor, the log temporarily "jumps" to another monitor.
Whether that can or cannot happen isn't always obvious.

Maybe logging to the default monitor (which may not even exist) is a
bad idea, and we should log to stderr or a logfile instead.  But
that's outside the scope of this commit.

Change cur_mon to point to the current monitor.  Create new
default_mon to point to the default monitor.  Update users of cur_mon
accordingly.

This fixes the periodical spamming of the default monitor by
usb_host_scan().  It also stops "log jumping", should that problem
exist.

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