Add support for GNU-compatible printf

This adds initial support for GNU find-compatible printf syntax. As far
as I can tell, it should implement all the specifiers, with the
following exceptions:

- Using zero as padding instead of space: GNU find isn't consistent with
  which numeric format specifiers support this.
- Specifying precision (`%x.yz`): I believe sparseness (%S)
  and seconds-since-epoch (`%A@` / `%C@` / `%T@`) are the only ones that
  allow this, making it only partially useful.
- Printing SELinux contexts (`%Z`): requires some extra dependencies and
  isn't within our scope.

uucore is added as a dependency, because it has several utility
functions that we need, the complexity of which is high enough that
using the extra dependency is quite helpful.

This also stubs out many of the specifiers for Windows, simply because I
have no idea how these should work there. (The defaults chosen should be
"good enough" for the moment, though.)

Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

.gitignore

@@ -3,5 +3,4 @@ target
 .project
 .cargo
 .settings
-test_data/links/link-f
-test_data/links/link-d
+test_data/links/link-*