Bug 5578: Self checkout by Login enhancement

Bug 5578: Self checkout by Login enhancement

Enables the library to choose whether to have patrons scan their barcodes for self checkout, or login
with username and password.  Uses 'checkpw' for compatibility with LDAP authentication.

Also introduces a few new system preferences to make Self Checkout more secure and manageable:

  SelfCheckTimeOut:      the number of seconds before the self-checkout login times out for a patron
  AllowSelfCheckReturns: indicate whether or not patrons can return materials via self-checkout
  SelfCheckHelpMessage:  user-configurable HTML to show specific text on the Help page.

Thank you to Marlboro College in Marlboro, VT for sponsoring and testing this development!

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>