This covers a few different things that were somewhat unrelated to starting to use filexfer but will make that transfer easier.

• Collects and isolates marshal/unmarshal code into packets.go
• Removing clamp and min functions that both did the same thing, and didn’t really contribute to more understandable code.
• Fix corner cases in os.ModeType → uint32 where we treat the various types as bitfields that can be OR’ed together rather than mutually exclusive values.
• a small bit of linting on request-server.go
• fileInfo can use the values in the wrapped FileStat, which we also don’t need to treat as an any.
• bring over the ls_formatting from PR-432, as this is an improvement beyond just bringing in filexfer as it extracts more UID/GID values for the various GOOS, supports setting UID/GID values from a FileStat, and also converts UIDs/GIDs to usernames and groupnames when possible. (Do we really want to use system-wide username/groupnames? Might be good to provide an ability to override this? Since request-server might be operating entirely disjoint from the underlying OS.)
• removed a few cgo build conditions that were not actually necessary
• verified that the code compiles for all GOOS supported in go1.16 (not verified that it works, just that it compiles)