Polish
Restore Spring Boot 1.1 logging behavior

Refactor LoggingApplicationListener and LoggingSystem to restore
Spring Boot 1.1 logging behavior. The LOG_FILE and LOG_PATH system
properties are now set before configuring the logger.

The `logging.path` property is now once again optional and will not be
used when `logging.file` is specified. The documentation has also been
updated to reflect the changes.

Fixes gh-2121
Fixes gh-2117
Substitute ${PID} variables with file logging

Update DefaultLogbackConfiguration to include OptionHelper.substVars
when setting the encoder pattern so that ${PID} is correctly expanded.

Fixes gh-2119
Unify console and file logback patterns

Update the file log pattern to write the thread name after the `---`
separator. The allows both file and console logs to be parsed in the
same way.

Fixes gh-2136