Support for multiple LDAP URIs for LDAP Authentication

This single commit implements storing multiple LDAP URIs in the hostname setting. They will be tried-out in succession on each log-in. There is a couple of catches, though:

1. The UI has not been adjusted to have some fancy input for those. Single line with space-separated URIs is used.

2. The connect attempt times-out after 1 second, so if there's lots of servers it might take a while. It's also not a configurable time-out, which may work badly for poor connectivity.

3. I had to modified the error handler function in TBGContext in order to have it not fail on function calls that use error control operator (the '@' thingie in front of a function). It does seem to be recommended way (according to PHP docs), but I'm not sure if it'll affect the rest of TBG in some way. This was necessary in order to test LDAP connectivity with ldap_bind function.