Replace misc. general functions with the ones from the cnc-gokit

[tomachalek]

General rules:

1. In case a function from a general package is not in cnc-gokit, move it there and upgrade cnc-gokit.
2. In case a local function is slightly different than the cnc-gokit one, then upgrade local code (i.e. avoit modifying cnc-gokit as much as possible)

Namely:

• fsops/fsops.go
• api/api.go
• general/collections/misc.go
• general/collections/multidict.go
• general/collections/set.go
• mail/mail.go (here replace EmailNotification with mail.NotificationConf, or define EmailNotification by wrapping mail.NotificationConf)

[tomachalek]

A note to the EmailNotification: To keep custom methods (e.g. DefaultSignature()) without moving them to the cnc-gokit too, please use a wrapping struct:

type EmailNotification struct {
    Wrapped mail.NotificationConf
}

func (enConf EmailNotification) DefaultSignature(lang string) string {
    if lang == "cs" || lang == "cs-CZ" {
        return "Váš CNC-MASM"
    }
    return "Your CNC-MASM"
}

// etc.

Once an instance of mail.NotificationConf is needed, you can just use myInst.Wrapped.

[tomachalek]

Already fixed