networkd: return 1 from successful event handlers
networkd: make metric of routes configurable

Now route metric can be configuted via conf file:

example conf:

[Match]
Name=em1

[Route]
Gateway=192.168.1.12
Metric=10

Test:
ip route output
default via 192.168.1.12 dev em1 metric 10

[tomegun: squash TODO update and reword man page a bit]
networkd: dhcp add vendor class indentifier option 60

Vendor Class Identifier be used by DHCP clients to identify
their vendor type and configuration. When using this option,
vendors can define their own specific identifier values, such
as to convey a particular hardware or operating system
configuration or other identifying information.

Vendor-specified DHCP options—features that let administrators assign
separate options to clients with similar configuration requirements.
For example, if DHCP-aware clients for example we want to separate
different gateway and option for different set of people
(dev/test/hr/finance) in a org or devices for example web/database
servers or let's say in a embedded device etc and require a different
default gateway or DNS server than the rest of clients.