client: wire up alternate_name

client: wire up alternate_name

Here we're exposing a public Client::walk (aka path_walk) so that the
user can inspect dentries (not something normally possible in POSIX).
We're going to skip exposing such an interface in libcephfs since
there's no reason to do that (who would use it?) except for testing.
Instead, a follow-up PR will add Client tests (for the first time, yay!)
that will exercise this code.

Also, ideally we'd also expose alternate_name via readdir results but
that is a bit more complicated since dirents do not normally refer to
external memory. So, just rely on Client::walk for testing for now.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>