Allow soft limits to be disabled

Set `DHLM` to "inf" and `DLLM` to "-inf" to skip configuration of soft
limits.

Rationale: Here are some reasons why users might want to disable the soft limits:
1. For single-axis systems soft limits can be specified inside the hard limits. This is not possible for multi-axis systems with more complex hard limits (e.g. not a simple switch but a pin touching some weird boundaries). In this case soft limits do not make sense.
2. Systems that can be calibrated by the user in different ways (e.g. home position at 0 vs. home position at 10000) might not want to use soft limits.