Untie logic for SCRIPT_NAME and PATH_INFO

SCRIPT_NAME is used as fallback for virtual-root when virtual-root is
unspecified in cgitrc and PATH_INFO is used as fallback for the query-
string parameter 'url' when the latter is unspecified. But until now,
the use of PATH_INFO depended on virtual-root also being unspecified,
i.e. it was impossible to use PATH_INFO when virtual-root was specified.

This commit makes the fallback on SCRIPT_NAME and PATH_INFO independent
code paths, i.e. it is now possible to specify virtual-root in cgitrc
while still using PATH_INFO (instead of rewrite rules) to get 'pretty
urls'.

Noticed-by: Jack Moffitt <jack@chesspark.com>
Noticed-by: LiKai Liu <liulk@cs.bu.edu>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>