userdb: Make lookups return a const pointer

userdb: Make lookups return a const pointer

This makes it more obvious that the returned pointer points to a
struct owned by the userdb, which must not be freed or have its
contents modified, and is only valid to dereference until the next
modification to the userdb's underlying hash tables (which in practice
means until the lock is released, because after that we have no
guarantees about what might be going on in another thread).

Signed-off-by: Simon McVittie <smcv@collabora.com>