1c80386 rpc: Generate auth cookie in hex instead of base64 (Wladimir J. van der Laan)

Base64 contains '/', and the '/' character in credentials is problematic
for AuthServiceProxy which represents the RPC endpoint as an URI with
user and password embedded.

Closes #8399.

Github-Pull: #8858
Rebased-From: 1c80386

8817d76 Add config changes to release notes (random-zebra)
988d428 [tests] Unit tests for -testnet/-regtest in [test]/[regtest] sections (Anthony Towns)
7b72630 ArgsManager: special handling for -regtest and -testnet (Anthony Towns)
51c4aa1 [tests] Unit tests for network-specific config entries (Anthony Towns)
30f3bab ArgsManager: Warn when ignoring network-specific config setting (Anthony Towns)
1bddffe ArgsManager: limit some options to only apply on mainnet when in default section (Anthony Towns)
ef505ea [Tests] Fix and re-activate rpc_users.py functional test (random-zebra)
0eacce7 [RPC] Add rpcauth startup flag for multiple RPC users (random-zebra)
e79ff3c [Tests] Fix and re-activate feature_config_args functional test (random-zebra)
3fcaaa4 Replace cookie auth in tests (random-zebra)
b907f33 rpc: Generate auth cookie in hex instead of base64 (random-zebra)
bf443a9 [tests] Use regtest section in functional tests configs (Anthony Towns)
7857bcd [tests] Unit tests for config file sections (Anthony Towns)
4cf2ee6 ArgsManager: support config file sections (Anthony Towns)
2658771 ArgsManager: drop m_negated_args (Anthony Towns)
4ee69b5 ArgsManager: keep command line and config file arguments separate (random-zebra)
4f416b8 [tests] Add additional unit tests for -nofoo edge cases (Anthony Towns)
5d1200b [tests] Check GetChainName works with config entries (Anthony Towns)
157da7c [tests] Add unit tests for ReadConfigStream (Anthony Towns)
23a4633 ReadConfigStream: assume the stream is good (Anthony Towns)
56ea59e Separate out ReadConfigStream from ReadConfigFile (Anthony Towns)
a3438a2 Test datadir in conf file exists (MeshCollider)
459c4ad [tests] Add unit tests for GetChainName (Anthony Towns)
0ab3e99 Move ChainNameFromCommandLine into ArgsManager, rename to GetChainName (Anthony Towns)
1ea7ce6 Globals: Decouple GetConfigFile and ReadConfigFile from global mapArgs (random-zebra)

Pull request description:

  It is now possible for a single configuration file to set different options for different networks. This is done by using sections or by prefixing the option with the network, such as:
  ```
      main.uacomment=pivx
      test.uacomment=pivx-testnet
      regtest.uacomment=regtest
      [main]
      mempoolsize=300
      [test]
      mempoolsize=100
      [regtest]
      mempoolsize=20
  ```
  The `addnode=`, `connect=`, `port=`, `bind=`, `rpcport=`, `rpcbind=`, and `wallet=` options will only apply to mainnet when specified in the configuration file, unless a network is specified.

  Also
  - fix cookie-based authentication for the functional tests, and re-enable `feature_config_args.py`
  - add `-rpcauth` startup flag, for multiple RPC users, and re-enable `rpc_users.py`.

  Backports relevant commits from:
  - bitcoin#8856 Globals: Decouple GetConfigFile and ReadConfigFile from global mapArgs
  - bitcoin#11829 Test datadir specified in conf file exists
  - bitcoin#12878 Config handling refactoring in preparation for network-specific sections
  - bitcoin#11862 Network specific conf sections
  - bitcoin#10533 [tests] Use cookie auth instead of rpcuser and rpcpassword
  - bitcoin#8858 Generate auth cookie in hex instead of base64

ACKs for top commit:
  Fuzzbawls:
    Tested ACK 8817d76
  furszy:
    utACK 8817d76

Tree-SHA512: 0d23dbf3d254b186a5378576601cf43f8322abe036b0b135a39b22e13ffa2e299cb1323160d87c7d8284e6aaa229c47f54c8a3a3ebf07cc298e644fb8bd69dc0