(read_from_string_index_byte): New variable.

(read_from_string_index_byte): New variable.
(read_from_string_index): Now counts characters.
(readchar, unreadchar, Fread_from_string): Changed accordingly.

(readchar): Read a multibyte char all at once
from a buffer, marker or string.
(unreadchar): Unread a multibyte char all at once.
(read1): Properly handle non-escaped multibyte chars.
They force a string to be multibyte.
When reading direct from a file, any multibyte sequence means
a multibyte string.
Insist on MAX_LENGTH_OF_MULTI_BYTE_FORM bytes when checking
for read_buffer full; this way need not check specially for multibyte.