Sort out / maybe fix viewer "logfile" option

[simon-linden]

Environment

Release viewer

Description

The viewer has a bunch of code for the "logfile" setting about logging to a custom file name. That, unfortunately, doesn't seem to work as expected. Look into it, sort out what's working and fix what is not, so that we can:

• set up command shortcuts to start the viewer and log to different files
• invoke the handleLogFileChanged() function when the setting is changed and switch files as expected

Reproduction steps

Use the log settings and look for files as you expect

[akleshchev]

Relevant code: LLAppViewer::initLoggingAndGetLastDuration

Also looks like it is supposed to be possible to change log file using command argument UserLogFile or a debug setting UserLogFile.

P.S. Currently viewer generates an uuid log for second instance, but that id isn't used anywhere else, probably should be used for crash reporting if we report crashes from second isntance

[mayconbekkers]

Opened a fix for this in #5674.

Built locally and verified that startup now respects --logfile / -logfile without changing the default SecondLife.log behavior. Also smoke-tested runtime log switching and a second instance.