Disable log writing to disk

[abitrolly]

To create less stress for HDD drives that write block data, it could be useful to only output log data to the screen. It can be possible if --logdir= with no argument meant to disable log writing. For people who need to write logs into current directory, a variant --logdir=. with dot at the end could be used.

[abitrolly]

The check for an empty string could be added here

btcd/config.go

Line 581 in 01f26a1

cfg.LogDir = cleanAndExpandPath(cfg.LogDir)

but processing requires stabbing initLogRotator call and log write calls. It is better if they could be abstracted into one class.

[abraithwaite]

I'd like to log to stdout/stderr for systemd/docker configurations.

Edit: The server does both, but I still don't want to log to disk. :-)