Use trim_end() to remove trailing slashes

All urls and paths read from cgitrc needs to be 'normalized', i.e. get any
trailing slashes removed, and the trim_end() function works out nice for
this purpose.

Removing the trailing slashes also happens to fix the case where the virtual
root should be a single slash; that case used to require specifying the full
hostname (including the http:// prefix), but now it can simply be specified
as /.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>