Refactor "process" package

[boogie-byte]

• All context-less wrapping functions (the ones without WithContext
  suffix) were moved into process.go since they all are the same.
• Call context is now passed to all underlying functions in
  *WithContext() functions.
• All common *BSD bits were moved to process_bsd.go.
• Process.Tgid() method lacked a WithContext counterpart, so
  Process.TgidWithContext() was added for uniformity.
• NewProcessWithContext() function was added since NewProcess() is
  used a lot throughout the module, and there is no way to pass a
  context to it.

This is a part of #761 effort.

[shirou]

I found this PR changes to return nil instead of empty value when returning error on various functions like

func (p *Process) GidsWithContext(ctx context.Context) ([]int32, error) {
	return []int32{}, common.ErrNotImplementedError
}

This may break application code. but checking error value at first is a golang-way,
so I think this is not become a problem on application.

I merge this. Awesome work, Thank you!