Also warn if bitcoind is configured to use a relative -datadir path.
Specifying paths relative to the current working directory in a daemon process
can be dangerous, because files can fail to be located even if the
configuration doesn't change, but the daemon is started up differently.
Specifying a relative -datadir now adds a warning to the debug log. It would
not be backwards-compatible to forbid relative -datadir paths entirely, and it
could also be also inconvenient for command line testing.
Specifying a relative -walletdir now results in a startup error. But since the
-walletdir option is new in 0.16.0, there should be no compatibility issues.
Another reason not to use working directory paths for -walletdir specifically
is that the default -walletdir is a "wallets" subdirectory inside the datadir,
so it could be surprising that setting -walletdir manually would choose a
directory rooted in a completely different location.
Backport of PR12220
https://github.com/bitcoin/bitcoin/pull/12220/
Completes T448