Platform specific bits

[richfitz]

In the last few commits there is some compiled code to access things on linux/macOS where R's file manipulation commands don't quite cover what is needed (currently: detecting if files are the same, if a path is a mount point, and getting a list of mount points). More will probably come.

However, the code that is in there will (a) not run on windows, (b) some is non-POSIX and needs some careful #ifdefing