Fix more ungetc bugs with encoding errors
* src/lread.c (infile): New variable, replacing ... (instream): ... this. All uses changed. (readbyte_from_stdio): New function, which deals with lookahead. (readbyte_from_file, Fget_file_char): Use it. (Fget_file_char): When misused, signal an error instead of relying on undefined behavior. (close_infile_unwind): New function. (Fload): Use it. (readevalloop): 2nd arg is now struct infile *, not FILE *. All callers changed. (read1): Handle lookahead when copying doc strings with encoding errors.
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