serial_bt, bluez: support MTU exchange in BLE reception

serial_bt, bluez: support MTU exchange in BLE reception

The central must respond to peripherals' requests to exchange the MTU
value. If it doesn't then the connection terminates. The central has no
choice whether this exchange happens or when it does, applicability is
the consequence of using a specific device (a firmware implementation).
This was first observed with RDTech TC66C energy meters.

Add support for the mtu= parameter in conn=bt/ specifications. A good
default value is yet to get determined. The connection type or the chip
in use are insufficient conditions AFAICT. Currently it is assumed that
users specify the central's MTU when their device firmware requires the
exchange.

Use the "warning" severity for the diagnostics message when peripherals
request MTU exchange while the central won't respond (due to lack of a
value to respond with).