So far we've always cleared the node directory when provisioning the node, but
while testing some plugins we noticed that pre-generating some files in that
directory is useful. This just adds yet another flag to `get_node` that
disables deleting any existing node directory.

While we removed the `satoshi` param in ElementsProject#3603 it appears that the
`fundchannel` plugin was still passing it to the `fundchannel_start`
call. This fixes up the help text. Notice that technically the help text
changes the param name, but since it was internally always called `amount`
this change doesn't break the API, the help was just wrong.

The spec states that invoices with an amount, but lacking a multiplier, should
be interpreted as integer Bitcoin amounts:

   `amount`: optional number in that currency, followed by an optional
   `multiplier` letter. The unit encoded here is the 'social' convention of a
   payment unit -- in the case of Bitcoin the unit is 'bitcoin' NOT satoshis.

Suggested-by: Stefano Pellegrini <@St333p>
Signed-off-by: Christian Decker <@cdecker>
Changelog-Fixed: invoice: The invoice parser assumed that an amount without a multiplier was denominated in msatoshi instead of bitcoins.

I was told that DSNs are not trivial to format, so this adds an example to
help setup the postgres wallet backend.