Netlink socket refactoring

- merge two send() methods into one
- use internal receive buffer instead of asking user to supply one
- move setting sequence number to MessageFactory sending code
- don't limit send function to Kernel as a recipient
- move adding NLM_F_ACK to the caller side
- getSocketPid -> getPid
- unsigned int -> unsigned

One part missing is refactoring receiveAck (b/161389935).

Bug: 162032964
Test: canhalctrl up test virtual vcan3
Change-Id: Ie3d460dbc2ea1251469bf08504cfe2c6e80bbe75