altosui: When switching log files, don't terminate log thread

The log thread automatically switches output files when the incoming
telemetry changes. Don't use 'close' for that as 'close' terminates
the log thread as well as closing the file. Create a new
'close_log_file' function which just closes the file.

Signed-off-by: Keith Packard <keithp@keithp.com>