xattr functions mark unwritten bytes as initialized when given a zero-length buffer

Summary

The extended-attribute functions in `rustix::fs` (`getxattr`, `lgetxattr`, `fgetxattr`, `listxattr`, `llistxattr` and `flistxattr`) write into a `Buffer` and then mark as many bytes initialized as the system call reported. When the buffer has zero length, the operating system treats the call as a size query: it writes nothing and returns the size of the attribute value or of the name list. rustix still marked that many bytes as initialized.

Passing a `Vec` whose length equals its capacity through `spare_capacity` gives such a zero-length buffer, and rustix then set the vector's length past its capacity over memory that was never written. This is undefined behavior reachable from safe code. With an empty `&mut [MaybeUninit<u8>]` the call panics instead.

From 1.1.6 these functions return an empty result for a zero-length buffer, and new `getxattr_query`, `lgetxattr_query`, `fgetxattr_query`, `listxattr_query`, `llistxattr_query` and `flistxattr_query` functions return the size. The 0.38 series did not have the `Buffer` API and is not affected.