Remove shared global state from goflags to make it reusable

[Ice3man543]

A lot of shared state is kept in the library which makes calling it multiple times from servers unusable.

We should remove global state and make library reusable.

An example causing panic when called multiple times -

var (
    optionMap           map[*StringSlice]Options
    optionDefaultValues map[*StringSlice][]string
)

func init() {
    optionMap = make(map[*StringSlice]Options)
    optionDefaultValues = make(map[*StringSlice][]string)
}