icmap: fix the icmap_get_*_r functions

Make the icmap*_r functions read from the specified map rather
than the global map.

Also include icmap_get_string_r() which seems to have been missed out.

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>