BMessage: Add {Add,Find}Strings()

They add a BStringList to/extract it from a B_STRING_TYPE field.
BPackageInfo: Make use of BMessage::{Add,Find}Strings()
package daemon: Add support for activation change request

* daemon: Handle new request B_MESSAGE_COMMIT_TRANSACTION. It activates
  and deactivates given sets of packages. The new packages must be
  placed in a directory in the administrative directory. The daemon
  moves them to the packages directory and the deactivated packages to
  a subdirectory it creates. It also save the old activation state
  there.
* Add private BActivationTransaction, describing an activation change
  transaction.
* BDaemonClient: Add CommitTransaction(), which sends a given
  BActivationTransaction as a B_MESSAGE_COMMIT_TRANSACTION request to
  the daemon.

Completely untested yet.