Add command help
Store the absolute path instead

This solves register_shutdown_function() not logging the errors with some
web servers.
In the called function, the working directory may have been
changed for the shutdown, so the relative path to logfile, is not correct anymore.
This will give you problems if you're using absolute paths in the config to start
with... but that's for another day.
Fix syntax error under PHP 5.4

ERROR: "Can't use function return value in write context"
Note that empty() is a language construct, not a function.
Still, this seems to work properly under PHP 5.5