bug #2757: change the opac login generation

bug #2757: change the opac login generation

This patch change the method used to generate a new login for a user when his informations are modified(memberentry.pl).

Before the login are generated with the first letter of lastname and the 9 first letter of the surname, and do not verify the
login already exist).
Now the login will be lastname.surnameX, where X is an incremented digit if the login already exists.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>