After several hours of googleing and testing I finaly succeded in authenticating squid3 and apache2 with local users (from /etc/passwd)
For squid you have to use the file /etc/pam.d/squid
account required /lib/security/pam_unix.so
Grand rights on pam_auth
Warning, depending on Debian version or OS version (32/64) pam_unix.so may not have the same location, sometimes it is in /lib/i386-linux-gnu/security/pam_unix.so.
For apache2 edit the file /etc/pam.d/apache2
# @include common-account
auth requisite pam_unix.so shadow
account requisite pam_unix.so
Then make a symbolik link of httpd on apache2
Add read rights on /etc/shadow file or allow apache2 service user (www-data) to read the shadow file
Now it works !