Add another way to produce formatted ustrings, this time using printf
syntax, by forwarding arguments to g_strdup_printf() and then copying
the result into the returned ustring.

This includes a private ustring::sprintify() function that by default
just forward its argument but can be overloaded to do something else.
In this commit, that is overloaded for ustring and std::string so that
their .c_str() is passed to printf instead, avoiding the ugliness of
users always having to write .c_str() in their own lists of arguments.

Note that the same lack of type safety as plagues printf() and all its
variants (in both C and GLib) applies here: the arguments are just
forwarded on, so if you include too few or the wrong types for the
placeholders you specify, you invoke undefined behaviour just as in C.

For reasons like that, C++'s preference of streams over stdio, and the
hope that we'll eventually get an actual nice string-formatting solution
in the C++ Standard, I don't go out of my way to shout about this in the
documentation. Users who really want sprintf() will find it, without us
having to shout too loudly about it and risk being seen as recommending
it more than anything else. It's here for those who know they need it.

#21