Improve handling of coding-system mnemonic indicators

Improve handling of coding-system mnemonic indicators

This fixes assertion violations when the mnemonic is
given as a string, and allows non-ASCII characters be
used as mode-line mnemonic of a coding-system.
* src/xdisp.c (decode_mode_spec_coding): Handle multibyte
characters as coding-system's mnemonic.
(display_mode_element): If decode_mode_spec returns a multibyte
string, display it as multibyte.
* src/coding.c (Fdefine_coding_system_internal)
(Fcoding_system_put): If :mnemonic is a string, use its first
character.  This avoids assertion violations if someone uses a
string as the mnemonic of a coding-system.