(Fkey_description): Add optional PREFIX arg.

(Fkey_description): Add optional PREFIX arg.
Combine prefix with KEYS to make up the full key sequence to describe.
Correlate meta_prefix_char and following (simple) key to describe
as meta modifier.  All callers changed.
(describe_map): Rename arg `keys' to `prefix'.  Remove local
`elt_prefix' var.  Use Fkey_description with prefix instead of
elt_prefix combined with Fsingle_key_description.
(describe_vector): Declare static.  Replace arg `elt_prefix' with
`prefix'.  Add KEYMAP_P arg.  Add local var `elt_prefix'; use it
if !KEYMAP_P. Use Fkey_description with prefix instead of
Fsingle_key_description.