alpm_db_get_pkgcache()
 package cache \endlink will be invalidated
 @param handle the context handle
 @param dbs list of package databases to update
 @param force if true, then forces the update, otherwise update only in case
 the databases aren't up to date
 @return 0 on success, -1 on error (pm_errno is set accordingly),
 1 if all databases are up to to date