Reduce minimum "non-standard" ISDCF part name length to 1.

Reduce minimum "non-standard" ISDCF part name length to 1.

Otherwise on macOS (at least) you can't type in any numbers, seemingly because
it checks on each keypress so e.g. you clear it, enter 1 and a bell rings because
1 is too small.  It's fine on Linux (a too-small number is corrected when you
press enter).