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