Backport bug 7517 fixes from trunk.

Backport bug 7517 fixes from trunk.

* coding.h (ENCODE_UTF_8): Remove "Used by ..." comment.

* nsfns.m (ns_set_name_iconic): Remove.
(ns_get_screen): Don't assign integer to f.
(ns_set_name_internal): New function.
(Vicon_title_format): Extern declare.
(ns_set_name): Call ns_set_name_internal.
(x_explicitly_set_name): Remove call to ns_set_name_iconic.
(x_implicitly_set_name): Ditto.
(x_set_title): Remove commet about EXPLICIT.  Call ns_set_name_internal.
(ns_set_name_as_filename): Encode name with ENCODE_UTF_8.
Always use buffer name for title and buffer filename only for
RepresentedFilename. Handle bad UTF-8 in buffer name (Bug#7517).