Fix debug dir on linux and disable logging by default

[nileshpatra]

Closes: #429

This PR does two things:

• Change log dir to ~/.local/state if XDG_STATE_HOME is unset which is as per the freedesktop guidelines
• Disables logging by default
  • Gomuks keeps logging events, and the size of the debug log can quickly shoot up if there are any crashes. Unless the user wants, it is not a good idea to keep dumping logs by default. However, logging would work just fine if DEBUG=1 is passed.

Tested this on top of 0.3.0 gomuks release.

[nileshpatra]

@tulir can I get a review on this, please?

[nileshpatra]

This is now accepted into debian, see https://tracker.debian.org/news/1442864/accepted-gomuks-030-1-source-amd64-into-unstable/

Any action or even comment about this would be useful for us to move forward to put this into debian testing, and also backport into current stable.

[tulir]

Might be better to handle it like all other configurable paths in main.go, but this is fine as an intermediate step.

Needs a few small tweaks though

[nileshpatra]

I've done the changes asked -- could you check now?

[nileshpatra]

Anything missing / any further review on this? If not, can this be merged?