[backport] Reject invalid wallets

Summary:
Backport of Bitcoin Core PR#10885: Reject invalid wallets

d84e78e [wallet] Specify wallet name in wallet loading errors (John Newbery)
a6da027 Reject invalid wallet files (João Barbosa)
3ef77a0 Reject duplicate wallet filenames (João Barbosa)

Pull request description:

This PR prevents loading the same wallet more than once in a multi wallet scenario. It also prevents loading with invalid files: non regular files or symlinks.

Depends on D987
Closes T190

Test Plan: make check && ./test/functional/test_runner.py

Reviewers: deadalnix, jasonbcox, matiu, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T190, T172

Differential Revision: https://reviews.bitcoinabc.org/D988