d*gettext doesn't allow domain to be NULL

[Minoru]

According to the manpage (emphasis mine):

The domain name is used to determine the message catalog where the translation is looked up; it must be a non-empty string. For the gettext function, it is specified through a preceding textdomain call. For the dgettext and dcgettext functions, it is passed as the domainname argument; if this argument is NULL, the domain name specified through a preceding textdomain call is used instead.

Meanwhile, gettext-rs expects Into<String> for domainname.

This isn't much of a problem for dgettext, because dgettext(NULL, msg) is equivalent to gettext(msg). However, there is no cgettext, so dcgettext must provide this capability.

This brings us back to #44: making the domainname an Option will hurt ergonomics IMHO, and this time, we can't weasel out of it by providing separate functions (or can we?)

[Minoru]

I wonder if Into<Option<Into<String>>> will work, i.e. will it accept None and "hello" and String::from("hello")? I don't have time to check it right now though; just writing this down.