Convert tincd path args to absolute paths

Since tincd chdirs to its own configuration directory and only then
resolves relative paths passed as command-line arguments (like --config
and --pidfile), statements like these:

	$ tinc -c confdir init foo
	…
	$ tincd -c confdir -D

didn't work properly.

Now we resolve paths to absolute right when we receive them, and only
then do chdir.