Add -walletdir parameter to specify custom wallet dir

Summary:
Backport 1/4 of core PR11466
(commits 0530ba0, 80c5cbc and parts of 8263f6a)

Note:
This PR first add changes to bitcoind.cpp, init.cpp and qt/bitcoin.cpp
(commit 0530ba0) then remove them in a later commit from the same PR
(8263f6a).
Unfortunately keeping this commits in different revisions would break
the cmake build when the wallet is disabled, in a way there is no easy
solution to prevent it. These 2 commits are mixed in the same revision
to avoid this. Because the mixed parts from 0530ba0 are reversion from
8263f6a, this should not make the diff more difficult to review.

Test Plan:

../configure --disable-wallet && make check
../configure && make check
cmake -DBUILD_BITCOIN_WALLET=OFF -GNinja .. && ninja check
cmake -GNinja .. && ninja check

Run:

mkdir <wallet_path>
bitcoind -walletdir=<wallet_path>

Check that wallet.dat and db.log files are created in <wallet_path>

./test/functional/test_runner.py multiwallet

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2312