* Changes
+ Add PptpProfile.java, PptpService.java.
+ Add L2tpIpsecPskProfile.java
+ Add PPTP and L2TP_IPSEC_PSK entries in VpnType.java.
+ Add the secret option to L2tpProfile.java and have L2tpIpsecProfile
extend it.
+ Add MtpdHelper to send common PPP options.
+ Add getGatewayIp() to VpnService().
+ Revise VpnService.reallyGetHostIp().
Patch Set 6, 7, 8:
+ Add L2TP secret flag in L2tpProfile.
Patch Set 9:
+ Add description to VpnType.
Patch Set 11:
+ Pass ipparam to mtpd.
Patch Set 12:
+ Add L2TP secret support
+ Fix string constants
