Avoid running win_is_writable() on non-IIS servers

[nbachiyski]

win_is_writable() is both expensive (creates temporary files) and must use the @ operator due to a PHP bug in Windows.

It's use on non-IIS servers can be avoided if we first check if IIS7 rewrite rules are supported on this server.

[nbachiyski]

First pass: [16634]

[nbachiyski]

(In [16635]) Add another pair of parentheses, so that the right-hand win_is_writable() isn't always called. Fixes #15616