[bp/r50353][SEC] installer: make locking the installer the default option; add warning to encourage use of secure login; (excludes portion of r30353 already backported to 6x in r50349)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/6.x@50357 b456876b-0849-0410-b77d-98878d47e9d5