huawei: rework probing and detection

Long ago there were problems where certain Huawei devices would
stop responding on various ports, and sometimes would crash
randomly.  The theory at the time was that touching the secondary
ports made the device angry, thus the plugin simply opened the
ports and listened for unsolicited messages.  But if the device
didn't send any during that 7 second period, MM would not detect
and secondary ports at all.  Plus, it was always a hack.

Instead, the new theory is that the device crashes if unsolicited
messages are enabled (^CURC=1), the secondary port gets touched,
*and* then closed and left for a while.  Fix that by turning
unsolicited messages off at probe time, on when the device is
enabled, and off again when the device is disabled like happens
for other modems.  Thus when MM first detects the modem, it turns
off unsolicited messages and the serial buffer on the secondary
port doesn't fill up and crash the modem.

Second, this allows us to simplify the probing logic quite a bit
so that we can probe all ports we find, but we still wait to probe
the first port so we can turn off unsolicited messages and get
hints about what port is the secondary.

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