Avoid trailing slash in virtual-root

When setting virtual-root from cgitrc, care is taken to avoid trailing
slashes. But when no virtual-root setting is specified, SCRIPT_FILE
from the web server is used without similar checks. This patch fixes the
inconsistency, which could lead to double-slashes in generated links.

Noticed-by: Wouter Van Hemel <wouter@duodecim.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>