call service.Context() after service.Init() on windows

[ploxiln]

service.Context() is called after Init() (and after Start()) on unixes,
so some users did not expect or test with Context() called before Init()

this was noticed in nsqio/nsq#1359
and is still needed along with nsqio/nsq#1361

cc @mreiferson @judwhite

[mreiferson]

Ahh right, thanks.

[ploxiln]

It would make sense to also move the call to service.Context() earlier in the unix variant, so it's also before Start() and after Init(), but I decided to leave this PR more minimal for now.

... and also that's theoretically a backwards-compatibility break for unix programs using go-svc that depended on Context() being called after Start() ... but it seems pretty unlikely that some project would use go-svc but not care that the windows case is broken ;)