Merge #11781: Add `-debuglogfile` option

Merge #11781: Add `-debuglogfile` option

5a7c09a test: Add tests for `-debuglogfile` with subdirs (Anthony Towns)
4158734 doc: Update release notes for `-debuglogfile` (Wladimir J. van der Laan)
2323242 test: Add test for `-debuglogfile` (Wladimir J. van der Laan)
cf5f432 Add `-debuglogfile` option (Wladimir J. van der Laan)

Pull request description:

  This patch adds an option to configure the name and/or directory of the debug log file.

  The user can specify either a relative path, in which case the path is relative to the (network specific) data directory. They can also specify an absolute path to put the log anywhere else in the file system.

  Alternative to #11741 that gets rid of the concept of a "log directory" by specifying the path for the specific kind of log, the debug log. Which happens to be the only kind of log we have at this point*, but a hypothetical new kind of log (say, an audit log) would get a new option. This has more flexibility than specifying a directory which has to contain all of them.

  \* excluding `db.log` which is internally generated by the wallet database library, but that one moves along with `-walletdir`.

Tree-SHA512: 4434d0e598dc23504e5c9e67fdbaef56db4f0fd490f9f54fd503e69d4dda9b5b69c539e1794ed841e72161b7b1dc3374d2f1193dd431b057566750e56fd8f24b