signmessage and signmessagewithprivkey don't work unless the address you use is of type legacy

elements-0.18.1.9

signmessage and signmessagewithprivkey only seem to work with p2pkh which is not the default in elements.

With a default address type this errors on signing:

```
elements-cli getnewaddress
AzpoUdjKavd8ZL4NTnaWt4bEYfVo4a5J42pDdAzCrzH3F7M5xk5iNSNCR3RfV9XndGEdnFv5uTs1EBmN

elements-cli signmessage AzpoUdjKavd8ZL4NTnaWt4bEYfVo4a5J42pDdAzCrzH3F7M5xk5iNSNCR3RfV9XndGEdnFv5uTs1EBmN "test"

error code: -3
error message:
Address does not refer to key
```

This will work if you use a legacy address type:

```
elements-cli getnewaddress "" "legacy"
CTEy43xUzkSCZGFnx2YjdoNpN6v8tVRUeBfyAb4N4K7UbqwNvaxjcxJJ5w3aeUtUbznpasycz1RnZksM

elements-cli signmessage CTEy43xUzkSCZGFnx2YjdoNpN6v8tVRUeBfyAb4N4K7UbqwNvaxjcxJJ5w3aeUtUbznpasycz1RnZksM "test"
IAEC8m8LC8DPUNVdKVnWlV00XqoYLWPWn83iP8ZAQ33ZKNyPnoU5cuDCpZCeKdpjqblgz9hKa9xKPW8Uhzue8Us=

elements-cli verifymessage CTEy43xUzkSCZGFnx2YjdoNpN6v8tVRUeBfyAb4N4K7UbqwNvaxjcxJJ5w3aeUtUbznpasycz1RnZksM IAEC8m8LC8DPUNVdKVnWlV00XqoYLWPWn83iP8ZAQ33ZKNyPnoU5cuDCpZCeKdpjqblgz9hKa9xKPW8Uhzue8Us= "test"
true
```

signmessagewithprivkey appears to work but verify stage fails:

```
elements-cli dumpprivkey AzpoUdjKavd8ZL4NTnaWt4bEYfVo4a5J42pDdAzCrzH3F7M5xk5iNSNCR3RfV9XndGEdnFv5uTs1EBmN
cSKSEM2nt24PRNMntCpExVhQbrmk5MffaNgWDS9zxWCt1yJgXvEF

elements-cli signmessagewithprivkey cSKSEM2nt24PRNMntCpExVhQbrmk5MffaNgWDS9zxWCt1yJgXvEF "test"
Hy5mGfJvFmLy6ZWXxxxLwymMvyOBI+vkFpFo8s+oWuFuFQiFN/Rb13L9ZjoUGWjJjGuvGDuhFfHx+Vgsl08xWqY=

elements-cli verifymessage AzpoUdjKavd8ZL4NTnaWt4bEYfVo4a5J42pDdAzCrzH3F7M5xk5iNSNCR3RfV9XndGEdnFv5uTs1EBmN "Hy5mGfJvFmLy6ZWXxxxLwymMvyOBI+vkFpFo8s+oWuFuFQiFN/Rb13L9ZjoUGWjJjGuvGDuhFfHx+Vgsl08xWqY=" "test"
error code: -3
error message:
Address does not refer to key
```