Resolve DOCUMENT_ROOT symlinks and standardize Windows slashing for various...

Resolve DOCUMENT_ROOT symlinks and standardize Windows slashing for various paths related to Multisite rewrite rule generation. props SergeyBiryukov, nacin, wpewill. fixes #23104 and #23073 for trunk

git-svn-id: https://core.svn.wordpress.org/trunk@23295 1a063a9b-81f0-0310-95a4-ce76da25c4cd