Update fputcsv() to escape all characters equally.

At present, backslashes have special case handling within fputcsv(): when one
is encountered within a field that's being escaped, escaping stops until the
next instance of the enclosure character is hit.  This can result in malformed
CSV.

Fixes bug #43225 (fputcsv incorrectly handles cells ending in \ followed by ").
Revert "Update fputcsv() to escape all characters equally."

On second thoughts, while the behaviour _is_ broken, this isn't the right fix.

This reverts commit 9b5cb0e8059b1e8bec096067491ed8d75f878938.