Use designated initializers for array initialization

In addition, make magic number OS_LAST independent of other enum
members. If we add another OS in future, we do not need to change more
than one line of code.
Merge pull request #178 from alexander-zimmermann/init-array

ack: Use designated initializers for array initialization
Merge branch 'next' of https://github.com/alexander-zimmermann/flowgrind into cleanup-fg_pcap