property_list_service: handle timeout condition on receive

this adds the new error code PROPERTY_LIST_SERVICE_E_TIMEOUT that
occurs if the given timeout is reached when trying to receive data.
notification_proxy: handle error conditions in notification polling thread
utils: make buffer_write_to_filename overwrite instead of append to existing files
userpref: fix leak and possible endless recursion in mkdir_with_parents
lockdown: Don't shutdown SSL during StopSession already or device complains
userpref: Use system wide pair record path and add config directory getter
lockdown: Remove pairing code which is obsoleted by usbmuxd's preflight handler
userpref: Use /var/lib/lockdown as the system-wide pair record path on Linux
lockdown: Send EscrowBag when starting service for automatic unlocking
idevicebackup2: Add argument to enforce a full backup from device
idevicebackup2: Add experimental support for using backups in the cloud
lockdown: Only sent EscrowBag if we are running in a secure session
idevicebackup2: fix possible crash due to use-after-free
idevicebackup2: suppress message when notification_proxy gets disconnected