Add state saving mechanism to support proc restart

Also...
+ stop daemons before getting server IP;
+ remove setForeground();
+ add the DBG flag for Log.d calls.

PatchSet 3:
+ add CHALLENGE_ERROR and REMOTE_HUNG_UP to VpnManager
+ broadcast new error codes in VpnService
+ check local IP change instead of dns change
+ move removeStates() to VpnService.onFinalCleanUp()

PatchSet 7:
+ add encryption flag to PptpProfile
+ PptpService and MtpdHelper are revised accordingly