make modbusino 56% moah awesome.

[karlp]

Fixes/tweaks/changes included:

• fix flushing. This is probably the most important. Earlier, the timing would often mean that uart.isAvailable() would sometimes fail, purely because it was between bytes. It needs to properly wait for a genuine clear space.
• support a tx enable pin, necessary for many rs485 transceiver chips
• support setting the slave id later, if for instance, you generate the slave id from a EUI48 chip, or similar
• support logical addressing (td: registers at base address 0x4000, rather than at 0)
• properly ignore packets not addressed to us
• more return codes for the various conditions
• added example compatible with the jpzomenta code
• added example showing usage of 485 transceiver and logical addressing

Reasons for being so slow to submit this: had to remove some timings specific to an internal part being used here :)

[karlp]

The end of the patch chain is clean. Git has gone nuts with the history somewhere, as you can see there's multiple copies of the same commit, without it ever being removed. I could spend a day wrestling git to turn this into three or four single commits to have fun with rebase and stunningly concise history, but I'm realllllly not very interested in wrestling git just for the sake of the history. You're also welcome to just compare master HEAD with your master HEAD, and apply it yourself.

[stephane]

There are many useful things in this pull request but too hard for me to dig in and clean each individual patch. I don't want to increase memory consumption (library and RAM) so I only intend to merge the most useful bits only.