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 ").
Merge branch 'PHP-5.3' into PHP-5.4

* PHP-5.3:
  Update fputcsv() to escape all characters equally.