rpc: Prevent dumpwallet from overwriting files

Summary:
Prevent arbitrary files from being overwritten. There have been reports
that users have overwritten wallet files this way. It may also avoid
other security issues.

Fixes #9934. Adds mention to release notes and adds a test.

Backport core's PR9937

Depends on D1165

Test Plan:

make check
./test/functional/test_runner.py wallet-dump

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1166