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

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

Backport of core PR11781

Depends on D2393

./test/functional/test_runner.py feature_logging

bitcoind -debuglogfile=<path_to_debug_file>

Check the debug file is at the given location

bitcoind -debuglogfile=<path_to_inexistant directory>

Check an error is output:

Error: Could not open debug log file