[LinuxCNC/linuxcnc Issue#577] Stepconf uses minimum step times at least for G540 causing failure to work with 3.3V parallel ports

Here are the steps I follow to reproduce the issue:

1. Run stepconf
2. select G540 parameters
3. G540 absolute minimum step time is used

Suggest all timing be double the drives specifications which are almost always minimums

G540 is especially bad because it specifies 1 usec minimum step time
but needs closer to 2.5 usec with 3.3V parallel ports

This is what I expected to happen:

Some timing margins

This is what happened instead:

0 timing margins

Information about my hardware and software:

LinuxCNC master (but probably every version)