Change "utf_8" to "utf8" in most identifiers.

Change "utf_8" to "utf8" in most identifiers.

Suggested by Miciah on #elinks.

What was renamed:
  add_utf_8                      => add_utf8
  cp2utf_8                       => cp2utf8
  encode_utf_8                   => encode_utf8
  get_translation_table_to_utf_8 => get_translation_table_to_utf8
  goto invalid_utf_8_start_byte  => goto invalid_utf8_start_byte
  goto utf_8                     => goto utf8
  goto utf_8_select              => goto utf8_select
  terminal_interlink.utf_8       => terminal_interlink.utf8
  utf_8_to_unicode               => utf8_to_unicode

What was not renamed:
  terminal._template_.utf_8_io option, TERM_OPT_UTF_8_IO
    Compatibility with existing elinks.conf files would require an alias.
  --enable-utf-8
    Because the name of the charset is UTF-8, --enable-utf-8 looks better
    than --enable-utf8.
  CONFIG_UTF_8
    Will be renamed in a later commit.
  Unicode/utf_8.cp, table_utf_8, aliases_utf_8
    Will be renamed in a later commit.