daemon: Fix log reload for relative paths.

If the argument to --logfile is a relative path, it is interpreted
as relative to the current working directory. In daemon mode, the
current working directory is changed to / during startup. Hence,
when para_server re-opens the log file after it received SIGHUP, the
logfile path will now be interpreted as relative to the the root of
the file system.

Fix this by remembering the original current working directory.
Opening "." as recommended in getcwd(3) is not an option here since the
whole point of changing the cwd to / is to prevent the daemon from
keeping the cwd busy.