Fix {LOGIN,LOGOUT}_URL when webroot is / Bug#1311234
[gnuoy,r=james-page,t=james-page] Fixup use of / for webroot.
[gnuoy,r=james-page,t=james-page] Fixup use of / for webroot.

Cherry pick from next branch for stable fix.
[james-page,r=gnuoy] Cherry pick fix for use of / as webroot.