Some metrics not available on macOS/Darwin

[tallpsmith]

When running pmstat on macOS, several metrics are not available under this OS:

1. swapd
   1.1. memory
   1.1.1. buff
   1.1.2. cache
2. swap
   2.1 in
   2.2. out
3. system
   3.1. in
   3.2. cs

Might be possible to combine this work with #2310