(Fwrite_region): Add 7th optional arg

(Fwrite_region): Add 7th optional arg
CODING_SYSTEM.  Move gap after a newline code if we are encoding
in a coding system which requires designation sequences to be put
at beginning of line.  Set coding.last_block to 1 before calling
a_write for an empty file.
(build_annotations): Code tuned up for handling
pre_write_conversion of a coding system.
(e_write): Delete code for handling carryover of code conversion.
It is now handled in encode_coding.
(auto_save_1): Supply 7th new arg CODING_SYSTEM as Qnil to
Fwrite_region.