Fix 0004465: Add config to disable 'save login' feature

To increase security, the administrator may want to prevent users from
using a 'permanent' cookie, thus forcing them to authenticate each time
they start a new session.

The new config option 'g_allow_permanent_cookie' enables this.

Porting to master branch of the following 1.2.x commits:

• 56986173ea3a2da12345acecb11afc7ab374696a
• 6a9f3a810dca575cc230808b780a80946acdcb73
• 568ee14305c4c6935b18137032d13de097841286