windows/sound.cpp: Rename CheckDeviceCapabilities()

**Describe the bug**
`CheckDeviceCapabilities()` in `windows/sound.cpp` does more than what the name suggests: It has side effects such as updating instance variables such as `lNumInChan`.

**Expected behavior**

Have a name which makes it clear that updates happen.

**Version of Jamulus**
d5d4691221b76bc648d605dac867aeaa561d0a52

cc @ann0see @pljones

Start in windows/sound.cpp by reading CheckDeviceCapabilities() and searching for its call sites and references. Choose a name that reflects its updates to instance state, rename the function consistently, and verify that no references to the old name remain.

Written by the indexing model from the issue text.