Fix more ungetc bugs with encoding errors

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.