Add --logdir option to specify logging directory.

This commit adds a new option, --logdir, which works in the same fashion
as the --datadir option.  Consequently, the logging directory is name
"namespaced" by the network as well.  This resolves the issue where two
btcd instances running (one for mainnet and one for testnet) would
overwrite each other's log files by default.

It also provides the user with a method to change the logging location to
non-default locations if they prefer.  For example, it enables multiple
btcd instances on the same network to specify unique logging directories
(even though running multiple btcd instances on the same network is not
the most sane configuration).

Closes #95.