To facilitate easier management by users and to
follow generally accepted standards, this PR moves
the following all to user home directory, subdir
.joinmarket :
joinmarket.cfg file
wallets/ directory
logs/ directory
cmtdata/ directory
commitmentlist file
User can override location with --datadir option.
An info message is added on startup showing location.
More on bug: when signing txs with SW and non-SW inputs, the
existing signature_form for non-SW inputs didnt pay attention
to whether the tx serialization was SW or non-SW style, so if
a SW-style tx serialization was passed in, it was kept in SW
form after passing through signature_form, resulting in an attempt
to create a signature based on SW style sighashing. The fix here
just ensures that signature_form always enforces the tx format
to be non-SW style, so allowing callers to sign() to get a valid
signature whatever they pass in.