(ITS#3717) gentlehup doesn't restrict writes as intended

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.2,2.3,HEAD
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (24.126.120.178)
Submitted by: hyc


I just noticed that daemon.c sets the global restrictops when it wants to
disable writes, but the global value is only tested in
backend_check_restrictions if no specific backend is provided. Looks like
daemon.c will have to set it in each individual backend to really take effect.