BTransactionIssue: String improvements
BRoster::Private::SendTo(): Actually support NULL reply

Unlike documented passing a NULL reply would just fail instead of
sending the message without waiting for a reply.
package_daemon: Root: Add IsSystemRoot()
Add watching support for installation location package changes

Can be requested/stopped via BPackageRoster::{Start,Stop}Watching().
The notification message has the what code B_PACKAGE_UPDATE and contains
fields "event", "location", and "change count".
HaikuDepot: Update list on installation location package changes

Very dumb implementation (just does a full update). I leave it to the
HaikuDepot hackers to improve it.