This flag can be used to specify multiple different exposure levels
for a training. There was some code already in tesstrain_utils.sh
to deal with multiple exposure levels, so it looks like this
functionality was always intended.
The default usage does not change, with exposure level 0 being the
only one used if --exposures is not used.
The --bin_dir option to tesstrain.sh is not useful, as $PATH does the
same job much better, so switch to relying on that instead.
This also makes the code a bit more readable, as it removes the need
to refer to binaries as COMMAND_NAME_EXE rather than just command_name.
The fontconfig initialisation hardcodes using Arial. However it may
not be available, whereas the fonts being used later will be, so use
one of them for initialisation instead.
The --fontlist argument to tesstrain.sh was always ignored, even if
the language had no specific fonts specified in language-specific.sh.
Change this behaviour so the --fontlist argument is used if no specifc
fonts are selected by language-specific.sh.
