Bring UTF8_{getc,putc} up-to-date: it's been a decade since 5- and 6-byte

Bring UTF8_{getc,putc} up-to-date: it's been a decade since 5- and 6-byte
encodings and encoding of surrogate pair code points were banned.  Add
checks for those, both to those functions and to the code decoding the
BMP and UNIV encodings.

ok miod@