Re-write S_utf16_textfilter() to correctly handle partial reads of UTF-16.

Re-write S_utf16_textfilter() to correctly handle partial reads of UTF-16.

Treat any (and all) octects after the BOM (or all, if there was no BOM) as
initial read data for the filter, and call it to convert them to the first
line, reading more if necessary. This correctly handles the "problem" that
UTF-16LE read as a line, on the assumption that it's ASCII/ISO-8859-*/UTF-8/etc
will be truncated after the first octect of the "\n\0" pair that is "\n"
encoded as UTF-16LE. This fixes bug #69678.
Read from the upstream filter in block mode, rather than line mode.