sd-rtnl: add support for vxlan rtnl attributes

This patch adds vxlan rtnl attributes to sd-rtnl
networkd: rename netdev variables

Rename the netdev variables. Remove tunnel_
so that it can be reused .
networkd: introduce vxlan

This patch enables netwokd to create vxlan

Changes:
Added:
	1. File networkd networkd-vxlan.c
	2. to netdev
	    bool learning
	    struct in_addr group
	    uint64_t vxlanid;
	3. VXLAN subsection and config
	   parameters
networkd: unref tunnel

Unref tunnel while shutting down
tests: add test-socket-util
tests: add test-fdset
tests: add tests to test-hashmap

add tests for:
  - hashmap_remove_and_put
  - hashmap_first_key
  - hashmap_last
  - hashmap_steal_first_key
  - hashmap_clear_free_free
tests: unlink temp file used in test