avoid leaking full directory path

[piramiday]

I think it would be useful to disable leaking the full directory path in the upper label.
if the server's owner decides to point updog to a certain folder, that should be the root of the webserver, full stop.

e.g. in the main example:

Directory: /home/sc0tfree

vs. something like:

Directory: /

I will never want to disclose the real path of the folder in which updog operates, but if somebody else believes otherwise, then this could be an option, rather than a hardcoded change.

EDIT: also, this "chrooted" behavior is more in line with python's http.server than displaying the full directory listing.

[wuzhaoqi1015]

can no longer agree

[felmoltor]

Implemented here: #67