Support terminal emulators that encode the Meta modifier as 8th bit

Support terminal emulators that encode the Meta modifier as 8th bit

See discussion starting at

 https://lists.gnu.org/archive/html/emacs-devel/2021-06/msg00034.html

for the details.
* src/keyboard.c (read_decoded_event_from_main_queue): For
raw-text encoed input, if Meta bit is encoded, apply the Meta
modifier to single-byte characters that have the 0x80 bit set.
For input encoded otherwise, if the Meta bit is encoded, remove
the 0x80 bit after decoding the characters.
(tty_read_avail_input): Reset the 0x80 bit only if Meta bit is not
encoded.
(Fset_input_meta_mode): Support 'encoded' as the value of META.
(Fset_input_mode): Document 'encoded' for META.
(Fcurrent_input_mode): Support and document 'encoded' as the value
of META.
(syms_of_keyboard): DEFSYM 'encoded'.