Reading MS signal metadata in rdheader

In general, it's useful to be able to read record metadata without reading the signals.

For multi-segment records, however, rdheader just reads the MS header by default (rd_segments=False), which means it doesn't read any of the signal metadata (e.g. signal names and units).

With rd_segments=True, rdheader reads the signal metadata and also all of the segment headers, which are not always needed.

For some applications it would be useful to have a middle ground of reading the multisegment header and the layout header, but not any of the segment headers. (Note that this is what isigopen does.)

I think this might make sense as the default behavior, since reading the signal metadata is a very common use of rdheader.

Start by locating the rdheader and isigopen entry points and inspect how their multisegment, layout, and segment headers are read. Check existing tests for multisegment records, then define completion as reading signal metadata and the layout header without reading individual segment headers, with coverage for the selected default or option.