Allow user to refuse the default problem solutions

Allow user to refuse the default problem solutions

Add new method SolverSolution::db2trans to change the solver's
transaction list to reflect the package database.  Use it if the user
refuses the default problem solutions.  Reinstate warning that this
could cause breakage.

Remove PrereqPage::OnMessageCmd, which is no longer needed (and which
disallows clicking Next if the "Accept default problem solutions" box
is unchecked).

v2:
Tweak MessageBox text