Merge #20003: net: Exit with error message if -proxy is specified without arguments (instead of continuing without proxy server)

9b4fa0af40cd88ed25dd77962235fbf268bdcaa7 net: Print error message if -proxy is specified without arguments (instead of continuing without proxy server) (practicalswift)

Pull request description:

  Exit with error message if `-proxy` is specified without arguments (instead of continuing without proxy server).

  Continuing without a proxy server when the end-user has specified `-proxy` may result in accidental loss of privacy. (The end-user might think he/she is using a proxy when he/she is not.)

  Before this patch:

  ```
  $ src/bitcoind -proxy
  …
  2020-09-23T00:24:33Z InitParameterInteraction: parameter interaction: -proxy set -> setting -listen=0
  2020-09-23T00:24:33Z InitParameterInteraction: parameter interaction: -proxy set -> setting -upnp=0
  2020-09-23T00:24:33Z InitParameterInteraction: parameter interaction: -proxy set -> setting -discover=0
  2020-09-23T00:24:33Z InitParameterInteraction: parameter interaction: -listen=0 -> setting -listenonion=0
  …
  2020-09-23T00:24:33Z init message: Starting network threads...
  ```

  `bitcoind` is now running *without* a proxy server (`GetProxy(…, …) == false`, `HaveNameProxy() == false`, etc.).

  Note that the "-proxy set" log messages above which the end-user might interpret as "good, my traffic is now routed via the proxy".

  After this patch:

  ```
  $ src/bitcoind -proxy
  Error: No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>.
  $ echo $?
  1
  ```

ACKs for top commit:
  laanwj:
    re-ACK 9b4fa0af40cd88ed25dd77962235fbf268bdcaa7
  kristapsk:
    ACK 9b4fa0af40cd88ed25dd77962235fbf268bdcaa7, I have tested the code.
  hebasto:
    re-ACK 9b4fa0af40cd88ed25dd77962235fbf268bdcaa7

Tree-SHA512: 4ba7a011991699a54b5bb87ec68367c681231bf5dcd36f8c89ff9ddc2e8d29df453817b7e362597e652ad6b341a22b7274be0fd78d435e5f0fd8058e5221c4ce

0 files changed, 0 insertions, 0 deletions