Enable extension loading from .user.ini when run as CGI.

Functionality is not included for other SAPIs because they either do not
support .user.ini (e.g. CLI) or they serve multiple requests and thus
do not support dl() (e.g. FastCGI).

There is also a bugfix to invoke zlist_clean, to ensure extension_lists can be
reused for the second round of extension appliations (since the head and tail
pointers have garbage in them).

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

(Available for PHP 5.3 too on request.)