win: Map ERROR_INVALID_FUNCTION to EISDIR

This error is raised when calling read() or write() on a directory.

A bit of googling turns up some cases where this error can be raised
that are not properly mapped to EISDIR, but are also cases that libuv
doesn't really care about, like the Password Manager API,
GetFirmwareEnvironmentVariable, or CreateTapePartition.

If libuv ever needs to handle these cases, then I suppose that the
ERROR_INVALID_FUNCTION->EISDIR mapping could be done directly in the
fs read() and write() functions, but doing so at this point seems
premature, as it makes the error code mapping a bit more messy.

Fixes joyent/node#4951

cc: @sblom @piscisaureus