Device/Driver/FLARM: use narrow strings if possible

Reduce overhead on Windows.
Device/Driver/FLARM: smaller string buffers

The FLARM dataport specification states that NMEA lines may not be
longer than 82 characters.  A buffer of 256 is wasteful; let's reduce
to 90 which contains enough margin.
Device/Driver/FLARM: convert task point name to UTF-8 explicitly

Pass narrow string to FlarmDevice::SetConfig().  Prepares for the next
patch.
Device/Driver/FLARM: purge illegal characters from task declaration

FLARM doesn't like to see an asterisk in a task point name, as this is
the end-of-line marker in the NMEA protocol (TRAC #3323).